It is rare to lose all Firefox bookmarks and without proof I would not be looking at AVAST as the culprit.
If Firefox was updated recently (you might not know if it is set to update automatically) this is much more likely the cause. Similar problems to this were reported when Mozilla updated to Quantum ie. FF v57 and I've found posts more recently saying the same thing has happened after other FF updates in the last year.
Google about your loss of FF's bookmarks and you'll find suggestions from Mozilla's support forum about how you 'may' be able to get them back. Unfortunately there is little information provided about whether those suggestions worked or not.
If you have any Restore Points then I'd try them and see if that gets them back for you. I do not rate your chances highly but it is worth a go.
The solution to preventing this sort of disaster is always keep backups of your bookmarks and copies of your backups too. It is easily done via FF's bookmark manager using the Export option.
If you ever need to restore the bookmarks you use the Import option but be aware that you'll probably lose all your bookmark icons until you revisit the bookmarked sites.
