This is to bizarre. One forum I go to, if I try to play an embedded YouTube video, Firefox freezes. I go to another forum and no problem, embedded video plays fine. If I go to YouTube itself, videos play fine.

Windows 8.1
Kaspersky Internet Security
Malwarebytes Premium

Firefox 41 (happened with earlier versions too)
Add-ons Manager (Extensions - all disabled)
Adblock Plus
DVDVideoSoft YouTube MP3 and Video Download
Kaspersky Protection
S3.Google Translator
The Addon Bar (restored)
Video Downloader professional

OpenH24 Video Codec - Always Activate
Primetime Content Decryption Module - Always Activate
Shockwave Flash - Always Activate
Foxit Reader Plugin for Mozilla - Ask to Activate
VLC Web Plugin - Ask to Activate

If I start Firefox in safe mode, and go back to the forum, it doesn't freeze. Plus, Firefox seems to be gobbling memory.