Unless you create a Mozilla account, and regularly sign into it so things like new bookmarks get uploaded, your profile can just disappear -- after years of not worrying about it, it's happened to me 3 times in the last couple of weeks. I mounted a month old backup archive and got most of it back, losing maybe a dozen bookmarks I'd added during that month. I then decided to get more proactive, backing up my profile every couple few days. [I should do it every day, but my memory is far from perfect, & it's not yet habit.]
To back up a Firefox profile you just 7-zip or zip 2 folders and store those 2 files somewhere other than those profile folders -- the 1st time I backed up the profiles I left the 7-zipped files in the same folder for simplicity, and when my profile was deleted for the 2nd time, those .7z files were gone as well. The folders with your profile are here -- if there's more than one profile folder use the one with the latest date:
C:\Users\ [UserName]\ AppData\ Local\ Mozilla\ Firefox\ Profiles\
C:\Users\ [UserName]\ AppData\ Roaming\ Mozilla\ Firefox\ Profiles\
You can also transfer your profile to another copy of Firefox by copy/pasting the saved profile folders, then using Firefox's profile mgr, or just copy the contents of one folder to another. The Firefox support docs say losing a profile can happen when Firefox updates, but only one of the 3 incidents I suffered occurred after an update.
