This is a tech question and i know it's not the place for it but i'm hoping someone ran into the issue and resolved it somehow. i googled to hell and back and can't find an answer. danbooru, rule34, e621, pornhub, etc, some videos show up blank but i can hear them. i use windows 7 home 64 bit, firefox 51 but recently upgraded to the current version to no avail. things i've tried

fresh firefox install
hardware acceleration on and off
flash on and off
clear all cookie, cache and history, everything
updated java
updated flash
updated graphics card software
disabled ad blockers
disabled all addons and plugins
restarted computer
safe mode

i have 16gb ram, up to date and above average graphics card (same issues with old card)
no viruses or malware
enabled and disabled virus software

it happens on chrome sometimes too. every website i searched said to try all of those things and nothings worked. when i download them and watch them they work but its too much to download all the ones that don't work.

example of ones with sound but no video

https://danbooru.donmai.us/data/__saltwater_crocodile_kemono_friends_drawn_by_haegiwa_gonbee__e19dc329c7112d59638591ad1b30e9db.mp4

https://danbooru.donmai.us/data/__original_drawn_by_kkamja__850296f910208f9aa7dd942d19996d37.mp4

https://iris.paheal.net/_images/9302611b212e2c1f1d9bc8a85fc657d6/3177634%20-%20Ms_Fortune%20Skullgirls%20animated%20beowulf%20magnta%20webm.mp4

if anyone has any ideas or has found a solution please share :D

Well, from examples I can say that those are videos taken directly from Twitter:
The first example is https://danbooru.donmai.us/posts/3537749
Source is https://twitter.com/PeToMa2738/status/1139320665213820929
And direct link to twitter video (because twitter converts gifs into video) is https://video.twimg.com/tweet_video/D8-t2o8VAAAcWPk.mp4
Which has MD5 of E19DC329C7112D59638591AD1B30E9DB
Which in turn gives us the post from danbooru completing the circle.
https://danbooru.donmai.us/posts?utf8=%E2%9C%93&tags=MD5%3AE19DC329C7112D59638591AD1B30E9DB&ms=1

So there shouldn't be any bad encoding or such in the process and twitter processes videos in way that they should work even with weakest and oldest hardware and software out there (I think they use Main 3.1, so, iPhone 4 era). Also all of those examples were MP4 with no sound, none were webm. So how can you hear sound? If they had sound, I could maybe see if your browser supports the audio codec, but not video codec, explaining the situation.

I do not see anything immidiately wrong with the files themselves. You can try accessing the tweet or video directly from twitter and see if those work.

Firefox 51 is from November 14, 2017, so if you actually had this old software and are still using Windows 7 for some goddamn reason, I would assume that firefox wasn't the only thing old in your system. Is this above avarage GPU intel HD? What is the processor that's most likely doing biggest chunk of the work, instead of listing RAM amount which in the context of troubleshooting this is pretty pointless.

What about drivers and updates? Have you disabled windows updates because piratebay said so? Are you still rocking nvidia 399.24 driver from September 10, 2018?

My point here kinda is that you have been doing kinda odd troubleshooting and aren't giving any kind of information to do any troubleshooting. Install all windows updates, install all related software updates, install all driver updates. If still having problems, give actual specs, not random information like amount of ram or subjective like "my graphics card is allright I guess" but "My GPU is Nvidia GTX 650".

You did say that videos play locally, but are you using VLC? Because VLC contains its own codecs in itself, so it usually works even if nothing else in the system does. I'm using K-Lite codec pack myself for my anime and such, but it also includes all the necessary codecs for all video and audio files, making them also work even with Windows Media Player.

i kept ff 51 because addons i used a lot started breaking after that version. then a few months ago, nearly all my addons broke so i had to update and find replacement addons. My windows is up to date and my drivers are too. yes i used vlc to watch the downloads

sorry i thought i posted the ones with sound and no video. i can't find them now. the troubleshooting is what people on other forums were saying to do. they sound like phone techs asking if you turned the pc on and off lol. i followed the instructions just to make sure. i didn't know what else to do.

my gpu is Radeon RX 560 Series. i say above average but i forget in today's times it's about average. I had a Radeon 260 series which couldn't handle 4k so i replaced it.

Sometimes i would get a MIME error but it would go away when i refreshed.

I just tried the twitter link and still no video.

what other specs would i need to show you? i can get them

Does this page show all the formats with green checkmark?
https://www.youtube.com/html5

they're all checked. im not sure that it matters but they are blue checkmarks. maybe a ff thing

So thus far:
- You say you have everything up to date and you apparently have computer that has parts that are under decade old.
- According to that page your browser should support and be able to play all HTML5 compatible codecs just fine.
- You have done pretty much all the basic level troubleshooting already.

You have yet to link examples with audio working. e.g. at e621 we do not have any automatic checks in place so sometimes badly encoded, broken or wrong container using videos slip by and are deleted later on as being corrupted.

I would also check if there are any codec packs installed on your machine, nuke those and try installing something like newest version of k-lite instead (there are other codec packs as well but most of what I know aren't updated). Sometimes software or games can bundle necessary codecs to play videos which then effect whole machine and in worst case scenario become absurd mess when everything is individually installed and not updated.