I have been using Firefox for over 10 years, I still keep stub v25 around in case I need to do a new install and use it to get the new tab button that someone decided back when it is not needed. A few versions back, as best I can tell, the 'show more tabs' arrow appeared on the right side of the tab bar. I am sure this is great for people that have 20 tabs open at once, but for most of us with 3 or 4 max it is not needed, and I believe that is what messed up the Hide Tabs CSS function, since it defies 'hiding'. Now in version 111.0, on some sites, when you go to read an article, a blue 'X' appears in the upper-right area, which apparently can be used as a "back' button. On some articles, if you use the back arrow instead of the 'X', which has been in use for years, the screen goes off into neverland. You can still scroll the article up and down, but it will not go back, the 'X' no longer works, and the only way out (that I found) is try to go to another site in the bookmarks bar (which may or may not work) or close Firefox and start over. This is unacceptable, and the last thing I want to do is go to Edge or Chrome. Please fix this mess!! The site where this shows up the most is kens5.com - a TV station in San Antonio. Just start reading articles on the front page, it hits about 90% of the time. Addons in my system are AdblockPlus, Forecast Fox, Disable HTML autoplay, Privacy badger, and Ugly Email.
You can check with the style wizards on r/FirefoxCSS for how to update your userChrome.css rules to account for the "List All Tabs" (Tab Manager) button: https://www.reddit.com/r/FirefoxCSS/
I don't think Firefox has started adding Back buttons or Close buttons into web pages. But some sites do muck with history in ways that complicate using the Back button. This is definitely an issue with sites that stack up intermediate positions within a page as you scroll down -- on those sites, Back doesn't seem to do anything and when you right-click the Back button it is full of the same thing over and over. I'm not familiar with other kinds of problems going Back. If the same problem occurs with add-ons disabled (Troubleshoot Mode), you can file a bug.