I love the slim design of the vertical tabs and your added functionality.
(For more inspiration: Look at some design solution from this (now obsolete) extension: https://addons.mozilla.org/de/firefox/addon/sidetabs/)
But when trying to move my window, I want to be able to mindlessly move to the highest point of my monitor and be able to drag the window.
In my attachment, you can see (in green) where you have to put your mouse over in order to be able to drag the window.
In red: This is a lot of horizontal real estate of potential window dragging capability. (It's not even that you are hovering on the search bar. This space really seems to have no functionality)
Some troubleshooting information:
133.0.3 (64-Bit)
Windows 11
2560x1440
1920x1080 (both don't work)
I like that you give the non-fullscreen windows an extra draggable corner (though maybe you should also adjust some extra pixels for again: horizontal real estate for dragging window).