Keeping Tabs on my Tabs – a quick Python hack

Published March 04, 2021 on Chandler Swift's Blog

I’m a bit of a tab hoarder. Firefox, thankfully, is pretty good about not using excessive resources for infrequently-accessed tabs, but when my Tab Counter extension told me I’d topped 300 tabs, I figured it was time to start a more serious diet plan. And what’s a diet without a scale? Here’s a script that I use to keep track of how many tabs I have open at a time, so I can measure my progress in the fight against my hoarding instincts.1

Firefox2 saves a sessionstore file that contains information about your windows and tabs. Unfortunately, that seems to be only saved when the browser is closed. However, Firefox also saves3 a $FIREFOX_PROFILE/sessionstore-backups/recovery.jsonlz4 with current data. This is the file I used. file tells us that .jsonlz4 is “Mozilla lz4 compressed data” – apparently they’re using a vaguely custom compression scheme to store this data. I didn’t have any luck stripping the header and decompressing it with the standalone lz4 tool, but Python proved workable. Once I have the data, parsing it is fairly straightforward. I wound up running the script once a day on a cronjob, frequently enough to give me that little extra prod to close my tabs, and one day in…I’ve closed almost 30!

#!/usr/bin/env python
import lz4.block as lz4
import json
from datetime import date, timedelta
import urllib.parse
import shutil

# Take a snapshot of today's file
shutil.copy(RECOVERY_FILE, f"recovery-{today}.jsonlz4")

with open(f"recovery-{today}.jsonlz4", 'rb') as f:
    assert == b'mozLz40\0'

yesterday=( - timedelta(days=1)).isoformat()
with open(f"recovery-{yesterday}.jsonlz4", 'rb') as f:
    assert == b'mozLz40\0'

def count_total_tabs(data):
    return sum([len(window['tabs']) for window in data['windows']])

# This is a unique tab identifier. However, it's not consistent, it seems to
# change when a tab is unloaded and reloaded (suspended?)

def find_matching_tab(needle, haystack, remove_from_haystack=False):
    # If they have the same docshellUUID, they're definitely the same
    for tab in haystack:
        if tab['entries'][-1][IDENTIFIER] == needle['entries'][-1][IDENTIFIER]:
            if remove_from_haystack:
            return tab
    # If they have the same URL they're probably the same
    for tab in haystack:
        if tab['entries'][-1]['url'] == needle['entries'][-1]['url']:
            if remove_from_haystack:
            return tab
    # TODO: if they share history, they may be the same

def find_matching_window(needle, haystack, remove_from_haystack=False):
    leading_contender = None
    leading_contender_matches = 0
    leading_contender_percent = 0
    for potential_match in haystack:
        matches = 0
        for tab in needle['tabs']:
            if find_matching_tab(tab, potential_match['tabs']) is not None:
                matches += 1

        match_percent = matches / len(needle['tabs'])
        if (matches > leading_contender_matches or match_percent > leading_contender_percent) and match_percent > .5:
            leading_contender = potential_match
            leading_contender_matches = matches
            leading_contender_percent = match_percent

    if remove_from_haystack:
    return leading_contender

def format_tab(tab, leading_spaces=0):
    entry = tab['entries'][-1]
    title = entry['title']
    if len(title) > 50:
        title = title[:50] + "…"
    url = urllib.parse.urlsplit(entry['url'])
    return f"{' ' * leading_spaces}{title} ({url.netloc})"

# TODO: persist these data
# TODO: graph these data
print(f"Yesterday: {len(yesterday_data['windows'])} windows, {count_total_tabs(yesterday_data)} tabs.")
print(f"Today:     {len(today_data['windows'])} windows, {count_total_tabs(today_data)} tabs.")

net_closed_tabs = count_total_tabs(yesterday_data) - count_total_tabs(today_data)
if net_closed_tabs > 0:
    print(f"Congrats! You closed {net_closed_tabs} tabs!")
elif net_closed_tabs == 0:
    print(f"Good work. You didn't open any more tabs today.")
    print(f"You opened {-net_closed_tabs} tabs. Dishonor on you! Dishonor on your cow!")

# TODO: tab entries have a lastAccessed field. Do some counting on that? (ms since epoch)

yesterday_windows = yesterday_data['windows'][:]
for i, window in enumerate(today_data['windows']):
    yesterday_window = find_matching_window(window, yesterday_windows, remove_from_haystack=True)
    if yesterday_window:
        print(f"Window {i} ({len(window['tabs'])} tabs)")
        print(f"Window {i} (NEW, {len(window['tabs'])} tabs)")

    tabs = window['tabs'][:]
    if yesterday_window:
        yesterday_tabs = yesterday_window['tabs'][:]
        yesterday_tabs = []

    opened_tabs = []
    changed_tabs = []

    for tab in tabs:
        yesterday_tab = find_matching_tab(tab, yesterday_tabs, remove_from_haystack=True)
        if yesterday_tab is None:
        elif yesterday_tab['entries'][-1]['url'] != tab['entries'][-1]['url']:

    if opened_tabs:
        print(f"  Opened ({len(opened_tabs)}): ")
        for tab in opened_tabs:
            print(format_tab(tab, 4))

    if yesterday_tabs:
        print(f"  Closed ({len(yesterday_tabs)}): ")
        for tab in yesterday_tabs:
            print(format_tab(tab, 4))

    if changed_tabs:
        print(f"  Changed ({len(changed_tabs)}): ")
        for tab in changed_tabs:
            print(format_tab(tab, 4))
    if not (opened_tabs or yesterday_tabs or changed_tabs):
        print("  (unchanged)")

for window in yesterday_windows:
    print(f"CLOSED: Window {i} ({len(window['tabs'])} tabs)")
    print(f"  Closed ({len(window['tabs'])}): ")
    for tab in window['tabs']:
        print(format_tab(tab, 4))

Potential enhancements that’ll probably never come since this was just a quick evening project to scratch an itch I had:

  • Clean up the code, Chandler!
  • Rather than snapshotting the (reasonably large) file each time, save the relevant bits to a database
  • Make the tab matching more robust. Currently if I’m visiting and click a link to, the script thinks I closed and opened However, since I do have access to each tab’s history, I could reasonably easily modify the script to process this correctly. I actually got something mostly working here, but stripped it out since it made the output logic a lot more complex.
  • Add a trendline for the n-day trend (alternatively, a streak–“You’ve closed as many tabs as you’ve opened for N days in a row!”)
  1. Generally, my usage consists of opening a bunch of tabs for a project, and closing them when I finish the project. This wouldn’t be that big of a deal, considering that a good project is generally in the 20-30 active tab range (generally consisting of documentation I’ll want to refer back to, obscure forum threads where DenverCoder9 mentioned my issue, API references, and of course some Stack Overflow tabs). However, this becomes a lot more of a problem when I’m swapping back and forth between several large projects, where I have a window for each. Currently, that list is:

    • DTANM - adding automated testing
    • Debugging screensharing on Wayland using Pipewire
    • Writing bare-metal Risc-V assembly
    • Fixing up an old super-tic-tac-toe server for better multiplayer
    • Finishing the Gallery app for AsteroidOS
    • Interfacing with MIDI instruments in Rust

    as well as a handful of smaller projects. One hope here is that this will be a nice kick in the pants to finish up some of those projects that have been sitting on the back burner for months. 

  2. I’m not looking forward to the day when this changes and it breaks everything that relies on this partially-undocumented feature. 

  3. It seems to save this file at least once a minute. I didn’t really care if it was once a minute or once an hour, since my script runs daily; I just didn’t want it to be “whenever you close your browser”! 

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