Firefox 89 (at least) took it upon itself to not only change how your settings handle scrollbars, but to also hide the ability to revert it back to using the system gtk scrollbars.
Here is how to get it back if you prefer to use your system gtk theme scrollbars in Firefox
In a new tab, type about:config
In the search bar at the top, enter
widget.disable-native-theme-for-content
This will bring the option back into view. Set the variable to False.
You can also search theme or scrollbar for various other settings. Or set widget.content.gtk-theme-override to the exact name of your system gtk theme (this will not affect Scrollbars, though and you still must do the fix above for that.)
For those that use the FireFox built in theme instead of gtk theme
If you find the scrollbars too thin in Firefox (As if they are a height gauge for the page instead of a tool to use), open about:config as above and search
layout.css.scrollbar-width.enabled
Double click to change the value.
Disable overlay scrollbars
Overlay scrollbars autohide the scrollbar when not in use. While this can be a good thing, sometimes it breaks UI or makes it difficult to access and use the scrollbar. It can be frustrating trying to get the thing to stop hiding while you are trying to use it.
To disable overlay scrollbars:
sudo nano /etc/environment
Add the following line:
GTK_OVERLAY_SCROLLING=0
Hit ctrl+x to exit, hit the Y key to say yes to save, then hit the Enter key to save under current configuration.
Alternatively, to disable overlay scrollbars only per specific application:
In terminal run (In this example Gedit- change the app name as needed)
GTK_OVERLAY_SCROLLING=0 gedit
OR in terminal run (Non-XFCE4 D.E.'s like Gnome, Budgie etc...)
gsettings set org.gnome.desktop.interface overlay-scrolling false