As for FireFox, what version of TestComplete 12 are you running and what version of FireFox are you running? For example, in my environment, I'm running TestComplete 12.20 and FireFox 53 (actually, technically, FF 53.0.2)... and I get your error. The reason being is that TestComplete 12.20 out of the box and the auto-update only supports up through FireFox 52. In order to be able to use later versions of FireFox, you need to upgrade to TestComplete 12.30 and make sure you check for recent updates to get the latest patches.
FYI, I tried applying the FireFox 53 patch to my TC 12.20 environment and I get the same error. It's possible that even an upgrade to TC 12.30 might still not work since the FireFox version is now up to 53.0.2
I have upgraded the Test Complete Version and Test Execute Version to 12.30 now.
I am seeing following issues with this version.
I am able to run test cases on Firefox.
Tool is opening Edge and IE browser also navigate to the site but after that nothing happens and Process stuck (stop) there only.
If you End the Run then no error is displayed.
I am also seeing that in all browsers except chrome if i stop run in between then tool just displayed stopping but it is not stopped after so long time too then i have to kill process through task manager.