This has been an ongoing bug with FF; I found search results here that are a decade old! And it's a big bug! Here's why: Unit: Linux Mint 20-C on a Vivobook using FF 80.0.1 (64-bit) 1: about:preferences > Applications > mailto > Firefox 2: about:preferences > Startup > Restore previous session 3: I have no other browser on my laptop (which is where this occurred). In testing by clicking an email link, it went into that new tab open loop (at a rate of about 2 tabs per second), because what is now obvious, is FF browser cannot be an email client; at least, not as it is, and has been. I tried to launch it in a variety of ways (e.g. private; restart with add-ons disabled), but no matter what, the loop would reopen the browser, and continue the loop. I had to restart my laptop in safe mode just to use it. So, I uninstalled FF and reinstalled it, launched it, and again it immediately looped uncontrolled. So, I again booted in safe mode, uninstalled FF, deleted all FF associated folders on my laptop, then reinstalled it. Fixed! Wow, that's just about as buggy as a computer can get! As the only browser, this created a serious conundrum. Thankfully I'm on Linux so I could install another browser brand if need be. https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows I finally found this page, after the fact, but this issue should not be a problem to begin with: Bug! Filing a bug report has been done multiple times over the last decade, with no resolution, so why do it again? Logic dictates that If this bug cannot be fixed, just eliminate this as a choice in the next version. Using FF (synced) on 4 different devices, I had to check them all and undo that setting! Fair warning to all. And most will not be as determined to stay with FF; they'll uninstall it, quietly, and just use another browser. FYI