This can happen because during the Firefox update process, the location of Firefox is temporarily moved from:
C:\Program Files (x86)\Mozilla Firefox\firefox.exe
C:\Program Files (x86)\Mozilla Firefox\updated\firefox.exe
You can verify the location of the running version of Firefox by starting Windows Task Manager and then doing a right-click and choosing properties for the Firefox.exe*32 process. If it shows C:\Program Files (x86)\Mozilla Firefox\updated\firefox.exe as the process location then there was a recent update.
As described on the Firefox website (https://support.mozilla.org/en-US/questions/936650) this means that there is an incomplete update pending.
The resolution is to do a reboot of the computer or VM and then Rapise should connect fine.
In a small number of cases rebooting the machine will not be sufficient and the only solution will be to uninstall and reinstall Firefox.