I was going through the suggested troubleshooting and it turns out disabling the theme I was using and using the default system one has fixed this issue. What a bizarre thing to cause this problem. So, subtitles are working and Firefox looks like crap. Oh well.

Hi Jota,

Thank you for sharing the solution here! Could you please tell what theme you used?

Hi Jota, Thank you for sharing the solution here! Could you please tell what theme you used?

Hi Denys,

The theme is called Dark Space - the best dynamic theme. I tried some different themes and still had issues with the subtitles not auto translating until I went back to default.

Now I've no idea if this is what's affecting everyone who is having issues with this on Youtube, I don't even know if they all use Firefox or not. I first highlighted the issue on the Youtube support forums because I was thinking it was probably a Youtube problem and I noticed thousands over there with the same basic issue of auto translate no longer working.

Rather strangely, today I tried auto translate on an Italian channel I watch - Stepsover, and their latest video had no translated subtitles but at least previous videos (going back months) were working. Same with another Italian channel - Storie di Mannu, the latest video had no auto translate but the older one's were working again.

Maybe there's some issue with Youtube and Firefox, but I've no idea.

Meh, the issue is still there. The auto translate is working on Edge but not Firefox. It auto translates correctly in a private window so what would be the issue on a normal window?

I tried the same video with Firefox in safe mode and the auto translate does not work.

So, to summarise, Youtube autotranslate video subtitles is not working in normal Firefox. It's not working in Safe Mode Firefox but they are working in New Private Window in Firefox.

What on earth would cause that?

Auto Translate to English for that video works fine in Edge, Chrome, Opera and Safari.

That sounds weird... Could you please try the following (in your normal window):

1. Please check if you have Enhanced Tracking Protection (ETP) enabled. You can do that by clicking the shield icon at the left of the address bar.
   • If ETP is enabled, try disabling it.
2. If the problem is not fixed, try clearing Firefox cache.
3. If it does not help, also try clearing cookies and data for YouTube (warning: you will be logged out on this website).

Did anything of the above help?

None of these things worked. Autotranslate to English worked in Edge today when I tested it.

When I click the autotranslate button and select English, the Italian subtitles disappear and then nothing appears. If I click the subtitles/CC button off and on again, it defaults back to the language spoken in the video (Italian in this case).

This is the Youtube channel name - "Storie di Mannu".

It's just an Italian couple who travel about the place. Can you try it on yours?

I have already tried and I face the same issue... But only in my main profile. When I use other profiles, auto-translated subtitles work just fine. I haven't been able to identify the cause, but could you please try creating a new Firefox profile and check if it also works for you? If it does, you'll hopefully be able to move to that profile while keeping all of your data.

I'm stuck on this bit - "In the toolbar, click the account icon." I don't see an account icon on my toolbar.

You should get the same menu when you click the Firefox menu icon and select Profiles at the top. If there is no such option, you'll have to use the legacy Profile Manager (about:profiles).

It works in a new profile with none of my previous profile copied over. I'll try copying it over and see.

Hope it will work... For reference, see Restore bookmarks, passwords, and data from an old Firefox profile. I recommend checking if the issue hasn't reappeared after copying every single file.

OK, I copied my old profile over, all 3.47GB of it and autotranslate is not working again. There must be something profile related but I have no idea what it could be.

Normal Mode didn't work, Safe Mode didn't work, Private Window did work and the new profile worked too (before I copied my old profile over).

That's some weird stuff for the devs to work out.

Could you please try copying files one by one? That way, we'll at least get a hint about where the issue lies. I asked several people, and none of them was able to reproduce it...

There's 3.5GB of them. That could take ages.

It's mostly about these few files. Could you try them with a fresh profile?

My god, after the trauma of having to watch more Youtube adverts than I've seen in the last decade I found the file that broke the auto translating.

prefs-9.js

The first prefs file, 'prefs.js' is 34kb then there's a sequence of prefs-1.js to prefs-20.js files which are all 0kb

prefs.js to prefs-8.js didn't break the translating, number 9 did.

Thank you very much! I can imagine your pain with the YouTube ads... I felt the same when troubleshooting :D

That's strange that the file is 0kb, though... If you open it with a notepad, what does it show?

It shows nothing in Notepad. I deleted the prefs-9.js file from the new profile and auto translate works again. Let me try copy and pasting prefs-10.js into the new profile folder and see if that breaks it too.

Edit.

LOL. I'm glad that you've been able to resolve this at least! And thank you once again for all your efforts.

I don't have prefs-<digit> files, so I'll have to go through the same troubleshooting process with copying files one by one...

OK, so I had prefs.js and prefs-1.js to prefs-8.js in the new profile folder, I copy and pasted prefs-9.js and auto translate stopped working. I delete prefs-9.js and auto translate works again.

I copy and paste prefs-10.js into the new profile folder after deleting prefs-9.js and auto translate was still broken but then I did the following - I deleted prefs-8.js and left prefs-10.js in place and auto translate worked again. It would appear that it's the amount of prefs files in the profile folder rather than any specific one that is causing the issue.

Truly bizarre.