By the way, is it possible to have FF and Waterfox coexist on this computer, and if so, how do I do it.
Yes, you can. When you install Firefox (or forks of Firefox like Waterfox), the programs install in one place. (On Windows, that will be under Program Files) Your user information, like your bookmarks, passwords and the like are installed in a profile directory somewhere else. (Under Win10 here, profiles get created under C:\Users<username>\AppData\Roaming\Mozilla\Firefox\Profiles. I override that, since the browser will let you choose where profiles are created, and put everything under C:\Mozilla\Profiles\Firefox.)
You can have more than one profile (and I do.) Firefox (and Waterfox) know where the profile they should use by looking at a file called profiles.ini, located in the same place as the profiles directory. You can also have more than one instance of the browser open at a time, as long as each uses a different profile. (The first one to attach to a profile locks it) You can specify which profile to use by invoking Firefox as "firefox -p " (I set that in a shortcut.) You can have more than one instance by invoking Firefox as "firefox -no-remote -p "
When you install Waterfox, it will create a separate profile. If you want it to use your existing Firefox profile, you can if you know what to use as after the -p above.
I looked at Waterfox a while back. It’s a worthy effort but I am not confident of its long term prospects. Waterfox deliberately uses a version of the Gecko rendering engine from before the changes that broke existing older extensions. Gecko is enormous, and nobody understands all of it. (I follow various Firefox bugs, and the first problem when a reported bug is confirmed is finding out where the bug is, and which Mozilla developers know enough about that part of the code to make the fix.)
I don’t know what development resources Waterfox has, but simply back porting security fixes made to base Gecko will be a challenge.
I used an ESR version of Firefox to buy time while I waited for compatible Quantum extensions to replace the ones I’m using. There are still a few gaps, but I found enough to let me use Quantum as my default browser.
I think you need to do some research on replacements for your critical stuff.
The password manager is still a hole. There are third-party password managers, but none that currently work in FF itself. (The last I knew, an API change that would permit one to work was under security review at Mozilla.)
You can get a dump of your password file using a third party utility from Nirsoft - https://www.nirsoft.net/utils/passwordfox.html, so you can at least see what they are. There is no current way to import that dump into another Firefox/Waterfox instance.
The visual changes you use Classic Shell for can be largely done in CSS, and the chap behind Classic Theme Restorer has a Github site for it. See https://github.com/Aris-t2/CustomCSSforFx