Hi, Yesterday I set up a new computer, installed Firefox and logged in to my Firefox account. Everything went well, my bookmarks appears, all (!) my extensions were installed, everything looked fine. Then I went about my way configuring unrelated Linux stuff, using Firefox as normal on both computers, no problems. But at some point, I suddenly noticed that on both computers, the icons for most of my extensions had disappeared. And sure enough, looking in about:addons, all that was left were five addons (Dark Reader, Simple Translate, uBlock Origin, Dictionary Anywhere, OneTab). And on both computers, including my main laptop (!), where I hadn't done anything other than clicking around in websites. And it only affected the extensions, my bookmarks, history, etc all is fine and synced between the computers. No restarting of Firefox, choosing different profiles in about:profiles (there was only one other, a mostly empty one), nothing got them back. And looking in ~/.mozilla/firefox/, sure enough extensions.json and all extension related folders had been changed, looking as if I never had more than these five extensions installed. And when I installed one of my usual extensions again, all of it's settings and customizations were back to default. So this is not just a case of having to reinstall a few extensions. All custom uBlock filters, all Tampermonkey scripts, etc, everything is gone. I tried looking around on the web for solutions, but it appears that there is no one who has had this exact issue before, and I couldn't find any indication that there is any way to revert such an error by Firefox Sync. To my luck, I have a secondary laptop where I should have a mostly up-to-date instance of my Firefox profile, from which I will now try to recover my extension data, but if that weren't the case, years of fine-tuned settings and customizations would be gone. As people rely on Firefox Sync to ''back up'' their Firefox data, instead of backing up their Firefox data "against" Firefox Sync failing like this, I'd think this is not something that ever should be possible to happen. '''At the very least, there should be some way to revert errors like this.''' Perhaps Firefox Sync could store backups either server side or client side before syncing deletions like this?