User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:51.0) Gecko/20100101 Firefox/51.0.1 Waterfox/51.0.1 Build ID: 20170130142126 Steps to reproduce: I have recently started experiencing browser crashes regarding use of webm video usage on web sites. Visiting any site that uses webm video will crash Firefox. I don't know when exactly this started, but I noticed it happening today. It had to have been fairly recent. I checked the same sites in another browser (Google Chrome) and they work fine. I'm curious if it might be related to Firefox itself. Actual results: All I've majorly changed recently on my PC is updating "K-Lite Codec Pack" from codecguide.com, but I've never had any side effects like this before. An example would be to visit this site here: http://www.webmfiles.org/demo-files/ You can also test on rule34hentai.net/post/list/lightning/1 Be warned, site has mature content and there's often lots of popups there. However, I've never been infected by any malware from them. Just a friendly precaution regardless, however. There are webm videos there that also crash Firefox. I'm simply trying to give as much information as I possibly can. Expected results: webm video is media content often embedded on many web sites. It should play normally like any other video content type, such as Flash, for example. I haven't had this problem in previous versions of Firefox before.

webm video continues playing fine outside of the browser... in Windows, such as in Media Player Classic. And they also play in Google Chrome, as I mentioned previously. I'm now lead to believe something recently changed in Firefox I wasn't aware of until today. Same crashing results and behavior in "Waterfox" (www.waterfoxproject.org). No, it never happened before in that browser either.

One more thing I can mention: The crashes only started today... and was first with "rule 34" site. After the first crash, Windows Defender claimed it caught something in one of Firefox's cache folders. I told Defender to remove whatever it found, but, I do also have Firefox set to clean up after itself. So whatever Defender would've found was most likely already cleared out with Firefox clearing itself. Firefox is set to clear history, cache, session... all of that stuff... probably cleared after crashes. Maybe something happened to the "rule 34" site and Firefox didn't like it. As I said, hadn't had crashes there or with webm before. Could there be some hidden setting that got changed as a safety precaution? But then, why is Firefox crashing instead of more logically giving prompts? In conclusion, webm videos play in Google Chrome and in Windows in media specific programs. I've run scans with Defender and Malwarebytes Anti-Malware and nothing's come up. Other web sites that use media, such as YouTube and Twitch continue to behave normally.

I just noticed mp4 videos will also now crash Firefox

If you have some crashes, type about:crashes and post some crash reports links (bp-...). And IMHO, you should clean your computer. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

turns out it was a custom theme related bug. https://rctgamer3.github.io/ff2-default/bugs.html author released an update recently and it appears fixed now. which I had suspected it was browser specific because they worked fine in Chrome and Windows.

(In reply to Loic from comment #4) > If you have some crashes, type about:crashes and post some crash reports > links (bp-...). > > And IMHO, you should clean your computer. > https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused- > malware By the way, my computer's clean, but I appreciate your concern. I DID state I ran the various scans. "That one site" was just an example for the videos. Please don't try to judge your users when they actually come seeking help from the source. Also, I tried "about:crashes" and it acted as if it doesn't exist.

To further clarify, upon doing some research... A change in a previous version of said theme did not agree with previous Firefox versions. https://addons.mozilla.org/en-US/firefox/addon/firefox-2-theme-for-firefox-3x/versions/ Current version of theme as of today appears to have resolved the problem. Problem was indeed related to video player controls with HTML5 video.