Hello, I’m lookin for particular add-on which may bring back good ol’ memories of that marvelous browser. I fell in love with Firefox long, long time ago. I was very satisfied with 3.6, but was afraid of newer ones. I finally installed 47, updated to 50, but now I don’t want to update it further; however I must - more and more sites refuses to load it contents correctly, antivirus is blaming me about expired certificates. But new-style UI is not convincing me. For FF 50 I had addon called “Classic Theme Restorer”, beacouse I really hate that all-in-one-button options list in right upper corner and it gives me a lot of customization possibilities. Unfortunately, I cannot find that addon anywhere now for recent version of FF, therefore I wish to ask You - do you know anything similar, which may spread all options as ‘File’, ‘View’, ‘History’ as separate at top of window and let me for similar possibilities in customization of my browser?
Turn on the built-in menu bar using the Toolbars list. Either:
- right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the “+” button)
- (menu bar) View > Toolbars
On Windows, tap the Alt key to activate the menu bar temporarily. - in Customize mode > Toolbars button (see: https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars)
To turn on the Menu bar or Bookmarks Toolbar, click it on the list.
For other customizations, what did you have in mind? The author of CTR maintains a repository of custom style rules that can modify the appearance of the toolbar area in numerous ways. You apply them using a userChrome.css file. Ready to dive in?
Oh, I see, Ubuntu FF - where im checking new version first - is a bit different than windows version and old-style toolbar is invisible until hoovered over title bar. Seems to be working… Anyway, for example colors are the thing what I wish to change. Font color of active and inactive tab, font of menu list, font at bookmarks toolbar, window color in general. With addon it was quite simple, but if there is possibility to use custom .css file and no addon in existance, therefore I should take a look into CSS. Heres a small example: 
Im gratefull for your quick and very accurate answer, thanks a lot.
And one more quick question - it is possible to have FF 50 and 60+ installed simultaneously without any conflicts?
You mean something like https://color.firefox.com/ ?
You can have multiple versions of Firefox installed, however you should run them on separate profiles to avoid conflicts (and it’s really not recommended to run an unsupported version of Firefox, but saying that often feels like shouting into the void). And that’s about as much as I’ll be willing to help on that.
I’ll try it, thanks a lot. Anyway I’m aware that outdated programs may be dangerous vulnerabilities to OS, however “newer” doesn’t automaticly mean “better”; and best example of that is Windows 10 and all its issues in every update. And nearly all I can lost of my personal information is bank account with huge loan. And the person which take that burden out of my shoulders will make me an unforgettable favor. But im grateful for your help.
-EDIT-
That color.firefox.com is nearly exactly what I was looking for, thanks a lot!