I wanted to submit this as a bug report to Bugzilla, but I can't find it anymore! An example of when I get an infinite loop is when I invoke a nonexistent method on an undefined object in my JS file. Another is when I rename the JS file pointed to by my HTML file being debugged and the debugger looks for the old filename. In both cases, I get the same error message and an infinite loop that makes my computer turn its fan way up until I close the tab containing the test html program. Source file shown it onloadwff.js The error message is: Paused on exception: Syntax error: element.queryselectorall: '*,:x' is not a valid selector. It is not okay to pause on an exception due to a syntax error in the debugger itself. It is not okay to go into an infinite loop due to a pause on an exception.