I think I found my kindred spirit! I have one Firefox instance presently at 3 windows and 3379 tabs and another with 10 windows and somewhere north of 2300 tabs. I wouldn't dare do this under Chrome/Chromium.
On my system at least, I've noticed that Firefox does get less responsive around 8000+ tabs. Mostly it's just the start up that complains.
Sometimes I'll get annoyed and mass-bookmark/close tabs (filed under date of closure). Do you ever do that or do you usually filter through things on an as needed basis?
For finding things I just use search in Power Tabs. Filtering down usually leaves me with a list I can scan through quickly. When it comes time to clean up I cheat by using that as well - filter my tab list, then select individual or groups of tabs to close. Generally I'll start by closing all tabs from news sites like the NY Times and WaPo under the assumption that they're probably stale, then clear out a bunch of HN tabs, and most often then switch to digging through the github ones to see what's actually relevant anymore. StackExchange and Reddit tend to get slashed pretty hard as well. Basically filter, viciously select, then close and I can blow away a thousand tabs in a couple of minutes.
The highest I've ever gotten was around 6000 on old Firefox for Linux and I did see some slowdowns then, but that was on a seven year old laptop.
Oh, there are kindred spirits here. I've recommended this on other tab-related discussions but Tab Stats is one of the few Firefox addons that have been worth using as a tab hoarder. Allows me to see which pages are open in multiple tabs and then deduplicate them if needed, or just looking at (or closing) open tabs based on domain.
(I'm not affiliated with the creator and it's not without its own annoyances, but still easily recommendable to anyone with thousands of tabs open)
I don't find it too tedious to go through and nuke things I know aren't useful long term (they're usually shunted to a separate throw away window for this reason!), but there's no easy solution for tab deduplication. What you've linked looks like it fits the bill!
Session restore is definitely not designed to scale up that high. I'm impressed that it works. If you're curious you should take a look at the session store data in your profile directory sometime, it's probably at least 50mb of javascript-encoded tab info and browsing history. (Yes, your per-tab navigation history is stored too)
Are you referring to the (previous.jsonlz4|recovery.(bak|json)lz4) files under sessionstore-backups? If so, you're very close: Uncompressed, they're around 40MiB. Each of my tabs doesn't store much history since they're usually opened and looked at--or saved for a later date with no further navigation.
I had assumed based on what I introspected from my profile directory the real bottleneck in my (ab)use was probably the JSON parser and/or my CPU. On my hardware, 8000 tabs seems to be the point where the hang detector fires, which I'm guessing is probably determined by dom.max_chrome_script_run_time? Sounds like you're telling me it's time for a CPU upgrade!
Anyway, I don't know if it's an interesting data point for you, but the only time I've had Firefox's session restore fail was if I killed my X session immediately after closing Firefox, presumably before it has a chance to save state. Even then, I've been able to restore it to an earlier state using some incantation of JSON from sessionstore-backups without losing much. Again, strictly a user-induced failure.
When I say that Firefox is the most stable and robust browser out there, that's not an exaggeration. If anything, it's an understatement. What's more, it's always getting better and improving. Since I see from your bio that you're a dev or contributor, I want to personally thank you for your efforts. Your work doesn't go underappreciated by us, even if we're less than kind to it. :)
I remember the browser wars. Consequently, I will always use Firefox. The internals exposed to idiots like me via the profile are pretty easy to reason about. I like that.
On my system at least, I've noticed that Firefox does get less responsive around 8000+ tabs. Mostly it's just the start up that complains.
Sometimes I'll get annoyed and mass-bookmark/close tabs (filed under date of closure). Do you ever do that or do you usually filter through things on an as needed basis?