Once a profile is touched by a later version, earlier versions will not want to use it. However, in Firefox 89, you can switch back.

Profile Manager Page

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least two profiles -- the one you want and the current one -- and could list many. (Each profile has two folders and one or more buttons. Do not use any Remove buttons!)

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

Look for another profile and check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, back on the about:profiles page, click the Set as default profile button for that profile so Firefox uses it automatically at the next startup.

Oh Thank God....Thank you, as well. That scared me to death!

Incidentally, can I.....can I back up my settings and the like somehow so that they can be used if I downgrade again? I seriously hate this new look, and unless there's a way to make it look like it did, I kinda wanna downgrade. I know how to backup my bookmarks (and will most decidedly do so to make sure my current bookmarks remain up to date even if I downgrade), but I dunno how to backup my settings just in case this way if worse came to worst, I would be able to just reload my bookmarks if nothing else, even if I couldn't switch profiles back to what I'm currently using, correct?

For that matter, this has kind of been an issue before with me, seems like every so often when the browser updates, it changes its look in a way that I just cannot stand. Is it possible to request that future updates have a way to change the look of the browser to look the way previous versions looked? Not like the changes to theme in the sense of a light and dark theme, but just a way to change the way the thing looks The visuals. Like, for example, the tabs here seem to just be free floating from the Toolbar, and it just looks....wrong to me. I hate feeling like my only option to make the thing seem more aesthetically pleasing is to have to downgrade— especially after this scare with the profile issue. Can that kind of thing be implemented? It'd certainly solve a big issue for those who find themselves unhappy with the way the UI looks.

Here's an article on backing up your profile folder: Back up and restore information in Firefox profiles

It would be awesome to have more configuration built in, but I am not very optimistic about that coming to pass. Firefox is built for numerous operating systems and languages, and there are thousands of automated tests running daily and for all the major and minor releases. New tests need to be written for every variation and this limits the flexibility that the designers are willing to build in.

Of course, we still have the power to modify the interface using a userChrome.css file, but that rabbit hole is a bit deep to start tumbling down at 11:32pm...

That would be a big help if they could allow people to use the look of previous versions of the browser. I would have thought that this would be something minor, since it's about the look of the browser, and they already have something similar with themes. For example, look at how firefox looks in 88.0.1 in terms of the tabs, which I will include a screenshot from in this post. Then look at the way the browser looks in 89.0 in regards to it. They might not seem like a big deal, but to me, especially since I only have the one eye, the difference is stark and in some ways more important than the themes. I'm all for a bright screen when needed, but the new browser's tabs are ridiculously bright compared to the previous browser's more laid-back grey. Between that and the floating look, it's kind of a mess in my view, hence why I say it looks wrong to me. I would kinda something as simple as being able to choose between the 88.0.1 look for the tabs and any newer versions wouldn't be that hard to implement, seeing as it could almost work as another type of theme, if necessary. Or at least so one would d think, right?

If you still have your old profile, first start Firefox from command line: firefox.exe --allow-downgrade and choose your old profile. If everything is OK, you can close and start normally. It worked for me downgrading from 88.0.1 to 78.11.0 ESR (see my answer on my own post https://support.mozilla.org/en-US/questions/1338596). I recommend using ESR, it has no new fancy update, just security updates.