Today I closed Firefox as usual. It wrote recovery.jsonlz4 and recovery.baklz4. (Sorry if I’m missing some details, I’m just a bit upset.) On the next launch, it didn’t show a window (most likely due to gWSL). I knew that after the next exit it would rewrite the session files, so I backed up .mozilla. (At that moment I didn’t realize recovery.jsonlz4 was already overwritten.) I killed Firefox and WSL, restarted, restored .mozilla, reopened Firefox — and it rewrote recovery.baklz4 with an empty recovery.jsonlz4. The session page showed nothing. I panicked, because my previous .mozilla backup is very old. I found out there may be a way to recover if I don’t launch Firefox before copying recovery.baklz4 to recovery.jsonlz4. So I closed Firefox, restored .mozilla again, checked that the .baklz4 file was larger, copied it over the .jsonlz4, and opened Firefox. It successfully restored everything. Why do I need to manually back up session files? Could they be versioned, or at least not replaced by an empty session? If the only tab is about:sessionrestore, that usually means something went horribly wrong, and it shouldn’t overwrite the previous session. Relying on third-party extensions doesn’t solve the problem. Replacing a valid session with a blank one leads to data loss. As you probably understand, this isn’t the first time I’ve lost data because of this behavior. This time I managed to save it, but previously I didn’t know these steps and lost my tabs.