If gitlab allows uploads by dropping files outside of the textarea, it might be a bug related to that (either in the browser or more likely in gitlab). This is standard for dragging urls or files into the browser from outside when the website isn't a drop target, afaik. You might be able to write a user script to disable it.

Alternatively, if you feel like rebuilding Firefox yourself, it looks like nsBaseDragService.cpp _might_ help you with preventing stuff from being dropped anywhere; I haven't read the whole file, but going by the name, overriding aCanDrop and mCanDrop to false might help. See lines 62, 84, 92. This _might_ allow you to continue dragging stuff around but disable dropping stuff.

[1]: https://searchfox.org/mozilla-central/source/widget/nsBaseDr...