Re: Where do I find the themes that I downloaded. They are not listed in the Add-ons page anymore.
Thank you for posting on Mozilla Support. We value and appreciate you as a user.
As far as I am concerned, I am sorry to say but you have lost your themes but my question to you is, did you uninstall Firefox then installed the new version or did you install the new version on top of the older version ?
If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available.
If this hasn't helped then also try to delete the addons.sqlite file.