not sure if you are looking for one but I have an extension that I wrote because I am similar to you and always had a hard time finding the tab I wanted. You might find it useful:
https://addons.mozilla.org/en-US/firefox/addon/tabist/
Source code: https://github.com/fiveNinePlusR/tabist
Why though? Is it all in the same context?
I can't stand 10's of tabs let alone 100's!!
Each instance of firefox has its own context. One is unsafe browsing, one is personal browsing, one is development-related browsing, a la Qubes. I used to have a problem with messy, unnavigable bookmarks so now I keep my bookmarks lean and rely on tabs to hold pages I may only need for a month or two at most. It's pretty quick to navigate to a tab with Ctrl+L
How do you create different "instances" of Firefox on the desktop? Or are you just talking about different windows?
You can also run multiple profiles in parallel, using something like
firefox --no-remote --profile-manager
I have that as an alias in bash, and a shortcut on my Windows desktop.
At least on my phone, Firefox Focus has helped immensely.