I restored add-ons, bookmarks, and favorites from another installation (Manjaro) by copying the contents of
~/.mozilla/firefox/??????.default into the correct folder of this installation.
I chose to reset firefox as detailed in their tutorial However, it did not fix the problem immediately. My add-ons (xmarks and LastPass) were not re-enabled until I restarted firefox for another time (refresh was the first time). Then, I discarded my xmarks settings and re-imported from a previous save. It was not clear to me if re-importing my xmarks settings or if just restarting the browser for the second time was the solution.