As per this thread:
https://www.reddit.com/r/firefox/comments/12hdx1v/mozilla_needs_to_launch_their_dev_tools_in_a/
Some sites (don't have any handy right now) detect and "sabotage" your browsing if you open dev tools. For example, they'll go into a debugger mode or become unusuable, telling you to close dev tools to continue.
The thread has some ideas about detection/prevention, tbh I'm not sure if this idea is possible but it would be useful.