Describe the bug
I have mapped Firefox to option-w but it only starts in safe-mode this way. Switching to Firefox when it already runs works though. Same behavior with Thunderbird.
Strange, when I press the "run script"-button this works, but when I hit the keyboard shortcut I get the same error as before. Does it really work for you when firefox is not running and you hit the keyboard shortcut?
Was having the same issue even with Hammerspoon. So this is not a BetterTouchTool bug. I found the solution in Hammerspoon's repository https://github.com/Hammerspoon/hammerspoon/issues/2256. Basically you cannot use 'alt' or 'option' as shortcut modifier as it triggers firefox's safe mode. I know... bummer