Sadly I have to edit my 5 star review because since some time in December, this extension no longer works properly. No matter what subtitles you select, the only subtitle that you get is a subscription demand for some subtitle site.
Very unfortunate, it was working perfectly before.

Update Feb 2024: Substital now works again but is subject to the opensubtitles API limit. Not ideal, but it's something.

I tried this on a YouTube video. The subtitles appear fine, but the menu doesn't work, so I could not change the background nor the size of the font. I tried it on Edge as well to see if it just clashed with other extensions, but it didn't work there either.

Please explain the permissions.
+Sub does not request "Access your data for all your websites": https://addons.mozilla.org/en-US/firefox/addon/plussub/

The only thing I can do is to add a subtitle to the video. Nothing happens when I press on the Subtital "Icon" on the video to access the settings. While on Google Chrome it works without any problems.

Need fixed:
1. No way to download different subtitle if first one is wrong, should be able to with the extension icon on toolbar.
2. Also should be an adjuster in the extension icon on toolbar so we can re sync subtitles.
3. Why have the logo on top of video, we know what we're using we did download it. Specially when it does nothing, you can't even click on it, for a menu to change or sync subtitles. Maybe even a source changer "opensubtitle, addic7ed, subscene........"

This add-on seems to be a really good effort. Unfortunately there are a few issues:
1. Most importantly, every time you pause or jump to a different timestamp it takes 5-10 seconds until it displays subtitles again, which is quite a dealbreaker for me.
2. The options menu doesn't stay visible (using Amazon Prime), even with the mouse cursor hovering over it. And somewhat funnily it will only show up again if the cursor leaves it.
3. A significant amount of time when looking for subtitles with the built-in search it will just show an error after loading for a while. I realize that this is possibly due to the external provider, but even then the error handling is suboptimal, since it throws you back to the main menu afterwards, meaning the whole process of searching and choosing a subtitle has to be done over and over again until it works.

Additionally, as I just noticed the timing seems to be off from the file. In my example the first line in the srt file starts at 00:00:19 seconds but after loading it into the add-on it shows the first line at about 00:00:09. Synchronization option is set to 0.

Also, having the option to add outlines and/or shadows to the subtitle display would be beneficial for visibility.