Sure, since vanilla firefox works, something in my settings contradicts to vimeo. But I have only addons and customizations that really needed for convenient use of firefox, I don't change a default if it's not required, so I can remove nothing from my settings.

I don't know how many vimeo-incompatible people are around. But I'm sure that there are no youtube-incompatible people

If you're interested in troubleshooting this, you can create a new profile using firefox -p and then following the prompts to create that new profile. Then you can add your addons and make your customizations a few at a time to see if you can narrow down when it occurs. Or it could be corruption in your profile itself (one of the annoying things that can happen with Firefox), which would be likely if you make all your adjustments on the new profile and vimeo still works. Then you can use that same command to switch back to your original profile so you don't lose anything.

I just know that all vimeo videos are not available on youtube (I usually check, because I find youtube a much better experience -- plus I can share those videos to my htpc easily whereas vimeo has always been hit and miss), so it could be beneficial to get it working.

I don't think that the profile corrupted since I recently reinstalled it (I have all settings in user.js). And if I find an offending setting, I will not remove it, because it needed for something.If I encounter such video, I'll download it.

This was only a suggestion in case you were interested in investigating it. If you were to find the culprit, it might be something that could be fixed without removing the change you desired by having that customization (maybe it is an addon with a bug, and the developer of the addon can fix it without changing the features the addon itself provides). Depending on what caused it would obviously dictate who you should report the issue to.

If you have no interest in investigating it, please disregard my suggestion(s)

@bassmadrigal: some days ago I couldn't login to transifex.com with Firefox using my credentials. Which was very annoying as that's where I store the Slint translation files so I couldn't manage them anymore with their web interface. No matter what, I couldn't find how to solve that issue.

I ended up creating a new profile, and that did the trick.

So yes, your suggestion is worth considering.

Alas, Firefox is now so huge and complex that sometimes it's hard to investigate an issue using it. I should refrain to say that sometimes this reminds me some software from Redmond, WA

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A while ago I migrated to SeaMonkey.
And while I admit occasional peak over the shoulder, I'm really contended how it serves me.
And many plugins work tho.
@Didier
+1
ever more projects do :[

By chance, I have discovered what in firefox settings brokes vimeo:This is a bug in firefox.

As a workaround one can use, for example, Autoplay Toggle extension, it creates a button that toggle "media.autoplay.enabled" setting.

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