Edit: 240321
on the "Zoom / Mirror" button displayed on YouTube Player at the bottom of the menu, there's a Mirror button. You just don't see it. If it's not FullScreen, you have to scroll down.

Now you don't have to scroll. Just need to update to v5.3.3. I let it all show.
=====
It's at the bottom of the menu. If you don't see it, scroll down.

Updated to v5.5.1
- Add Shortcuts for mouse
- Add option to enable/disable fixed shortcuts

Add Zoom, Move, Reset with mouse
Using the shift key, ctrl key is not suitable, there will be other problems.
You can adjust the speed in the options.

Edit: 240319
I just now understand your problem.

Must delete or change shortcuts in Manage Extension Shortcuts.
check: https://imgur.com/a/DIpmWP7
======

Updated to v5.5.1
- Add Shortcuts for mouse
- Add option to enable/disable fixed shortcuts

In option > enable shortcuts for > try to disable Fixed Shortcuts
If it doesn't work, try disable everything again.
Finally, if it still doesn't work. Try deleting in Manage Extension Shortcuts too

Reply to me again if there is still a problem.

Updated to v5.4.8
- Fix autoplay problem

I'll inform you, this one can't be fixed.
In theater mode, there is a problem when pressing rotate for the first time that it doesn't fit properly (while in default view / no problem in fullscreen) which is caused by the width and height of the image in theater mode.

2023/03/15
Ok, how is that account different from the others? I tried switching my google account but it didn't have a problem. Try changing the language or changing the theme, no problem.
I don't know what the cause is, I can't fix it. If you know the cause then please reply me.

//===============================
2023/03/14
I've checked both Windows and Mac in the latest version (v110.0.1), but it doesn't have a problem when pressing the zoom button and the menu doesn't pop up.

Please remove this add-on and install it in the latest version (Upload Today v5.4.7)
*** Not update. Remove it > Close Firefox > Reopen Firefox and install the add-on ***

I don't know what caused it. But I reworked the code sequence of the zoom function. Hope it solves your problem. If the menu still doesn't pop up Try using shortcuts instead.

//===============================
2023/03/13
HI. i have checked. It still works normally.

Can you try disabling all addons to just this one? If it works, it means some of your addons are in conflict with this one.

If it works then please reply me. I want to know which addon it is so I can check and fix it.

If not, what version of firefox are you using?
Previously, there were people who had problems with this zoom function as well because they were using Firefox Nightly, some versions that had problems.

2023/02/19
HI. i have checked. It still works normally, both the mirror and rotate buttons.
Or do you mean the shortcut doesn't work? maybe some addon use the same shortcuts
How to Manage extension shortcuts >>> https://support.mozilla.org/en-US/kb/manage-extension-shortcuts-firefox

if it's still not work. Can you try disabling all addons to just this one? If it works, it means some of your addons are in conflict with this one.

If it works then please reply me. I want to know which addon it is so I can check and fix it.

thanks.
if you have any problems, feel free to tell me.

Added (Next version I have plans to add option to hide button icon on youtube control bar and use only shortcuts)

v5.3.7
- change default icons
- add option to change icons in the add-on Preferences (5 icons per button)

i have tried deleting and reinstalling. it works without problems
please try checking the add-on preference to see if the value of "Adjust the Zoom/Mirror" still exists (normally this value will be set at start)

Try to update to version 5.3.2

- Add option to adjust value for zoom, x,y and crop in the add-on Preferences
- Fix some issue for long press +,- button