Are you on Windows 10 or Windows 11? With Windows 11, Firefox opts into floating/disappearing scrollbars by default (system-level "Always show scrollbars" setting). Not sure if Windows 10 does it, too.
Regarding width, my Firefox follows the standard width on Windows 10, but there might also be some hidden settings to adjust Firefox if your system uses narrow bars. I don't know the names, unfortunately.
You might consider posting this problem on the Mozilla Support site. They have a team of support volunteers to troubleshoot malfunctions with your currently installed version of Firefox, while this site is focused on idea submissions for future versions of Firefox (looking ahead anywhere from 8 weeks to many months). Here's a link to the new question form (for Windows/Mac/Linux): https://mzl.la/3y1vQUQ
