If I understand what you're saying, Firefox inherited everything correctly with these minor styling differences:

New Firefox has larger/wider tabs and the collapsible sidebar does not work, why?

Collapsible sidebar definitely is not a built-in feature. How were you doing that before? In particular, did it require start-up scripts in the program folder at

C:\Program Files\Mozilla Firefox\

For more information on where to check for some files you might not have brought over, see:

• Help article: Customizing Firefox Using AutoConfig
• My article: https://www.userchrome.org/what-is-userchrome-js.html

Hello, The collapsible sidebar (bookmarks) and tab changes was a CSS file I made and placed in the Chrome folder. I did run across a few files changed in about:config and have no idea why or how that happened. One had something to do with turning on, off Chrome/CSS files. After changing that back to true, my small tabs came back but not the collapsible sidebar.

I don't really have a theory on why the same code on the same OS on the same version of Firefox works in one case but not in another.

Do you recall where you got your code? Sometimes it helps to go back to the source and check for updates or review any tips on possible dependencies.