Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Could you give a sample URL and steps to reproduce the problem, i.e., what to click, etc.?

I didn't see the problem again in Safe Mode.

As for a URL, simply visiting a site like this enough: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/display/Stage.html

I was presented with the aforementioned error three times and pressed Dismiss All every time. The fourth time, I was unable to interact with the window and had to kill the program.

If it does work in Firefox Safe-mode then disable all extensions (Tools > Add-ons > Extensions) and then try to find which is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Ah, thanks; it seems FlashFirebug has gone from being less useful to actively antagonizing me. Away it goes. Cheers.