Windows 7 - so version 115.18.0 is the last version which can operate on a Win 7 OS. I'm trying to find out if there was an old bug on this, but regardless you cannot update whilst using a Win7 OS. Does the issue still occur if you restart Thunderbird in 'Troubleshoot Mode' ? What addon extensions do you use? I'm wondering if there is a conflict due to an addon. F as short cut will go to next message in folder Ctrl+F is shortcut to Find in this message So it sounds like your computer thought you had pressed either of the above. In which case please make sure your keyboard does not have a dirty contact for the Shift and Ctrl keys. Maybe you have got a Win7 setup interfering like a sticky key setting. Take a look at this link: https://support.microsoft.com/en-gb/windows/make-the-keyboard-easier-to-use-in-windows-7-c74e835a-dce9-a545-fae0-1dc7497e534f