uBlock Origin is just required for me to block ads and other annoyances. Generally I also use it to block the annoying cookie popups instead of clicking any of their buttons instead of opting for a different extension, like I don't care about cookies.
I also almost always have Tree-Style Tab, though I've found that it can cause Firefox to be non-performant on memory-constrained devices. This extension gives you a tab sidebar (using a similar interface element like the bookmarks sidebar) that organizes your tabs in a tree. I will often have hundreds of tabs open normally, and this makes them more manageable, as I middle-click everything to open things in a new tab, particularly when I'm researching. It gives a nice rudimentary tree view of what content related to what, like if I'm on a Wikipedia dive or TV Tropes dive.
I also usually have Tampermonkey, as I usually want to make a website more accessible. A lot of the time, it's simply so I can autofill usernames in a text box, because the website might have the login flow split between pages, and Firefox does not recognize or allow me to select an autofill the username for me. Other times, it's to automatically click through that annoying Microsoft login flow "Keep me signed in? (Don't ask as often)".
For watching YouTube, I use the SponsorBlock addon, because sponsored segments have become very annoying, though some creators have been able to make them actually fulfilling to watch.
On Android Firefox, I also get Disable Page Visibility API. This allows me to use YouTube in Firefox and even background the app and listen to music that way. Plus, with uBlock Origin, I am also able to block ads. I also sometimes install this on different profiles for situations where I believe the web app may try to detect my visibility, such as an online coding test for job opportunities where it will not want me to change windows to look up answers or type code into a compiler (this saved me during a C++ multiple choice exam which asked many times "Which of these is invalid C++?" and "What is the output of [this complicated code block]?").
The below is for my work computers only, because I generally don't need them otherwise.
I will additionally have Firefox Multi-Account Containers and couple that with Simple Tab Groups as I have 6 logins, with one particular site requiring 3 of those logins. I essentially configure the URLs I need to be in certain groups, define one container for each, and then have a couple of default tab groups set as sticky groups that will allow all containers to stay as those containers so I can use multiple sites side-by-side. The interaction with Tree-Style Tab can be a bit janky, especially when configuring new catch tab regular expressions, but usually disabling and re-enabling both Tree-Style Tab and Simple Tab Groups will fix that. Mostly I do this, because my company refuses to fix their SSO breaking due to weird cookie issues, and it's pretty nice to be able to clear cookies for a particular tab group easily.
To clear cookies, and because I do that very often, I use Cookie Quick Manager. I can clear a tab group's entire cookie set by having a tab selected in that tab group, then using "Delete current Context Cookies" and usually that will fix my login issues, and also keep my logins to other sites. For my personal use, I generally don't need to clear cookies, or using a private window, clearing per-site via the web console, or clearing in settings is good enough.
Since I do a bit of web troubleshooting, Modify Header Value is pretty nice. I can call API endpoints that require a subscription key from my browser, even using the web console, and I don't need to worry about figuring out if my headers are correct. I can also get my team to get this add-on so they can do their basic troubleshooting. For my personal computer, I'm much more free to create ad-hoc scripts to test things that I can save somewhere, as sending others scripts in my organization also comes with implicit hours of training and coaching on how to use them (and our leadership has been very sensitive to explicit training hours to get everyone up to speed).
Finally, User-Agent Switcher and Manager for the sites that are built for Chrome, but have worse performance or broken features on Firefox, yet they work fine when Firefox sends them a Chrome user agent. Thankfully, I rarely have a use for this on my personal computer.