Perhaps a little tweaking is in order. Shockwave and Java were outdated and needed re-installation on mine! Took a couple of efforts with restarts but it is obvious that it solved my problems! Firefox Mozilla Toolbar 'Help' > 'Firefox Help' > lower right side of page displays link to Plugin Update review....

Thanks for the response, but I make a point to keep all my plugins and apps up-to-date by using FileHippo, Secunia and information from a variety of blogs (Brian Krebs' in particular for security-related issues). That plugin check you referenced indicates a few that are unknown (including MS Office 2010), but all the Java, Flash, Shockwave and Silverlight plugins (plus a few others) are recognized and up-to-date. This didn't show up until v15 was installed as an upgrade, not a fresh or clean installation.

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

• Don't 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

Thanks for those suggestions, but so far nothing below the nuclear option seems to bring that functionality back. I tried starting in Safe Mode, disabled all extensions (I use the default theme) and turned off hardware acceleration -- I didn't reset the toolbar or button customizations at that point before testing to see if I'd regained the functionality, though. Unfortunately, the frame source popup remained a blank window.

Next, I restarted FF in normal mode, turned off hardware acceleration and disabled all extensions, then restarted and looked to see if the functionality had returned, but no luck there (and no surprise, either).

So, it would seem the next step is to reset the toolbar and controls in order to rebuild a potentially corrupted localstore.rdf file in my profile. One question, though -- does that step need to be taken with extensions disabled, or should it be done with all the extensions enabled, or does it make a difference either way?

You can check for problems with the localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf
• https://support.mozilla.org/kb/Toolbar+keeps+resetting

Can you display the page source via the location bar by prefixing the URL with the view-source: protocol?

• view-source:https://support.mozilla.org/en-US/questions/936109

Are any relevant view_source.* prefs not the default on the about:config page?

• http://kb.mozillazine.org/about:config