My only real complaint on switching from Chromium to #Firefox as my primary browser was the wispy little hide-and-seek scrollbar. With the help of this guide from the creator of the #CustomScrollbars extension, though, I got my nice fat scrollbar back! https://github.com/WesleyBranton/Custom-Scrollbar/wiki/How-to-enable-wide-scrollbars-on-Firefox
Here's how I did it:
1. Open Advanced Preferences by navigating to "about:config" without the quotes. Click "Accept the risk and continue."
2. Search for "widget.gtk.overlay-scrollbars.enabled" without the quotes. Double click this setting to change to "false."
3. Search for "widget.non-native-theme.scrollbar.style" without the quotes. 0 is the system default scrollbar style, so if a different style is desired double-click to change to a number between 1 and 5. 0 worked on my #ArchLinux system. For more explanation, see Heading 3 on the linked guide.
4. Search for "widget.non-native-theme.scrollbar.size.override" without the quotes. Double click and change to a number. 20 works for me.
And that's it! I now have a chonky scrollbar that stays in place without needing the Custom Scrollbars extension, but on other systems and configurations one may need to install the extension and play around with settings, including disabling "automatically hide scrollbar" in the preferences of the Custom Scrollbars extension. (These preferences can be reached by navigating to "about:addons" without the quotes, going to Extensions > Custom Scrollbars, and clicking the three-dot menu to the right of the enable/disable toggle button. That one took me a minute to find lol.)