It looks like the Profiler (Performance tool) is the one that is opening. Its keyboard shortcut is Shift+F5. Is it possible something is sending that combination to Firefox? Maybe something that tries to reload the page (F5 reloads and Ctrl+F5 reloads bypassing cache, so Shift+F5 would be unusual)?? Until that is sorted, you could try disabling that tool to see whether it makes any difference -- here's how: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''devtools.performance.enabled''' and pause while the list is filtered (3) Double-click the preference to switch the value from true to false