Freezes firefox on startup when using 'Prepend a symbol' feature witha large number of tabs about auto-tab-discard

[pallaswept]

By a process of elimination I have narrowed this down to an issue with the "Prepend a symbol to the discarded tabs (e.g. 💤 or ⏻︎) (if possible)" Feature. Reproduction steps follow:

It may be relevant that I have Firefox configured to load ALL tabs at startup (browser.sessionstore.restore_on_demand=false) - I have a powerful machine that can handle it just fine, and I don't want to wait around every time I click on a tab from the saved previous session which has not been loaded. I have not altered the setting to load them 'lazily' (browser.sessionstore.restore_tabs_lazily=true as per default), so if the session contains 1000 tabs across 5 windows, I am not loading 1000 tabs simultaneously, but all 1000 tabs will eventually be loaded when I start.

I did not always have this issue, I did not see this issue occur until I had 10 browser windows and over 500 tabs, so a session with at least 10 windows and at least 600 tabs should be used for this process.

• Configure Firefox to load the previous session on startup, and to load all tabs (the above setting in about:config)
• Enable the 'Prepend a symbol...' feature by putting a character in that box (I went with the ⏻︎ character since I use a monochrome theme/userChrome - probably best to do the same, for reproduction to be identical to mine)
• Discard all tabs - It must be large number of tabs (in the region of 600+ tabs)
• Note the titles' change with the character inserted as intended
• Exit Firefox
• Start Firefox
• Firefox will appear to freeze (but is actually just taking some 60+ times longer, to start - as in, a session which normally takes 30 seconds, will take >30 minutes, and the browser will be unresponsive the entire time
• Note that in spite of all tabs being loaded due to the abovementioned setting, many of them will have the prepended character, as though they are discarded, even though none should have been discarded yet because the time limit has not been reached.

Steps to recover and work around this bug:

• Wait for the faulty session to eventually become responsive. It will take dozens of minutes on a fast system. Possibly hours. It will be a LONG time, sorry!
• Remove the character in the abovementioned option, so that tab titles will not be modified when discarded. Don't forget to save settings
• To ensure that the addon does not discard tabs while we follow this process, disable the addon temporarily - the aim here is to have zero discarded tabs / zero tabs with modified titles
• Select all tabs in the window, then refresh them all (so as to un-discard them)
• Repeat the above for all windows, so that all tabs are now loaded and not discarded
• Re-enable the addon
• Discard all tabs
• Note that tabs are discarded, but titles are unaltered
• Quit Firefox
• Start Firefox
• Note normal startup

Steps to confirm that this feature is the cause:

• Disable the addon temporarily (to allow us to ensure all tabs are loaded / not discarded)
• Select all tabs in window, reload them all to ensure they are all loaded / not discarded
• Repeat for all windows (repeating the above process to ensure a direct comparison of behaviour)
• Re-enable the addon
• Re-enable the 'Prepend a symbol...' feature by putting a character in the textbox and save your settings.
• Discard all tabs (same process as above)
• Note the titles changing when the tabs are discarded
• Quit Firefox
• Start Firefox
• Watch it effectively freeze

So obviously, there's some kind of problem with loading tabs from a saved session, which have the modified title. It seems to only occur under certain other circumstances, or a mix of them, those being:

• 10+ Windows
• Greater than 500 discarded tabs (at least, maybe it needs 512 or 600, I can't say.)
• Disabled loading tabs on demand (ie, load all tabs in the session, immediately on startup)
  but any or all of these criteria can be met, and the browser will function normally, even with ATD running, and the only change is to prepend a character to the title.

Hope this helps you to narrow down the code that is to blame here. I now have a workaround, so I an enjoy this addon and it's more pleasant UX, but I really do like the 'prepend a symbol' feature, so it would be awesome if we could get it fully working. Please let me know if there is anything I can do to assist.