I had to downgrade to Version 3.3.4 because the Addon doesn't work on some sites it used to work with. Example 1: "heute.de" (reroutes to zdf.de) videos show a black screen, when viewed in fullscreen mode. Example 2: I sometimes watch videos via the Plex Web App and since the newest version the Addon doesn't seem to register the player anymore and stays in 16:9 in fullscreen mode.
Both sites work fine with the downgraded version.

EDIT: regarding Plex. I just tried it again, even deactivated all other firefox extensions except UltraWideo and when I use the Plex Webb App Player with Version 3.4.3 and load up a Movie or Series from my server, going fullscreen only shows the video in 16:9 with black bars on the side.
Doing the same thing with version 3.3.4 and it works as intended and stretches it horizontaly, so no more black bars. Not sure what else I can do to help in this regard.

Works fine but begs for a review every time you update with no way to disable it.

It worked once, but now does nothing.

I have an ultrawide monitor, but get to watch movies cropped. :(

Not great. It just zooms in, doesn't really fix the aspect ratio and doesn't work with a 32:9 monitor. Crops everything on top and bottom with wider monitors. Can see it working well with 21:9 ones though. Giving it 3 stars because I can see it working for most peoples' monitors.

When it works, its great! But lately Ive had trouble with 4:3 videos (or anything less then 16:9), and randomly it disables its self, or changes what mode its in.

Does what it says it does, but the shortcuts are very confusing.

I cannot get the pause toggle to work with shortcuts, making the extension very annoying to actually use.

Works, but it doesn't have the upscale mode that I need. Upscale seems to fill the screen horizontally from edge to edge, but it can cut off the top and bottom of the video if it's too tall. I'd rather upscale fills the frame vertically, and leaves black bars on the side if it has to. The frame should never be cut off. This is an issue I noticed with Expanse on Amazon Prime. The aspect ratio seems to be non-standard.

The extension has been working great. Except that it seems to be slowing down youtube videos to the point that they have to load (buffer) frequently while watching. I noticed this because I almost have Gigabit speeds on my internet. So I turned it off and the issue stopped.