Verify a couple of things:
1) Firefox IS actually running and present in the Task manager in Windows
2) Firefox has not been updated recently.
My guess is #2 happened. I looked online and see that Firefox 72 was JUST released yesterday. Currently, TestComplete only has patches available through Firefox 71 so, if you are running 72, it's probably not being recognized. You will need to roll back the Firefox version until Smartbear makes a patch available.
Best practice is to turn off automatic updates for browsers so that you have control of your environment.
https://support.smartbear.com/downloads/testcomplete/firefox-patches/
Here you can already download Firefox 72 - even though it is not possible to update automatically in TestComplete
