a single breakpoint is set in a function called by a menupopup's onpopupshowing attribute, in form myFunc(event, this). after the breakpoint hits, an immediate F5 Continue is issued, ending debugging. after 1-2 seconds venkman/firefox are frozen. if an F5 is not issued, ie while trying to inspect vars etc., then after 1-2 seconds the same freeze happens also. venkman 0.9.88.1 and today's trunk build.
Can you provide more specific steps to reproduce? With the information in your description alone, I can't reproduce this, nor track down what the problem is. Furthermore, does the same thing happen with 3.6.8 and/or the last Fx4 Beta?
from a very quick test, it did not happen in 3.6 with my extension, but i can definitely recreate it in: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b5pre) Gecko/20100827 Minefield/4.0b5pre. a better str: 1)start the debugger. 2)set a breakpoint in browser.js, function contentAreaClick, line 9056. 3)rt click on an new/empty tab and wait for the debugger to stop at the bp. 4)wait 5 or so seconds, or expand a variable object. i can recreate at will, sometimes the debugger doesn't even get to the bp before the hang. very minimal extensions, just domi, console2, addon compat reporter. only thing in console is a venkman css warning; there are no cpu cycles expended. let me know if there's some other diagnosis to do, though i think going back to various betas for regression is probably not too efficient..
closing, as this no longer happens, but still no debugger as bug 614557 hasn't gone anywhere.