OK, yes there’s already been an issue opened with Mozilla for the geckodriver issue.
While we wait for a fix, there is a workaround:
Go to Tools -> Options
Scroll down and click on Selenium
Where it says “Binaries” add “/usr/bin/firefox-esr”
Click OK to save the changes. It might give a warning box saying that it couldn’t save because of “null” but it should still have saved. You can double-check by closing and re-opening the Options window and make sure the changes are still there
Try opening Firefox via manual exploration, and it should work