A while ago Firefox introduced a, in my opinion useless, feature to get a preview of the webpage you can navigate to. With a long press of the left mouse button on a url you can get a preview. For some reason I trigger this by accident. So after the x-th time I accidentally triggered it, I now turned it off. This happens when you long press one of the tabs in the edit screen:
More information: https://support.mozilla.org/en-US/kb/use-link-previews-firefox
This in itself is super annoying, but it is nothing you can do about (not 100% sure if you can block this for urls on the same domain). I wanted to let you know that when you do this, after releasing the mouse button you can still drag the tab to the left/right. You are only supposed to drag the tabs when you are holding the left mouse button, but it is possible to this without holding the left mouse button.
I turned off this feature in Firefox, so problem solved for me.
Another way to trigger this (not sure if the same happens in other browsers):
Hold left mouse button on a tab (keep left mouse button pressed). You can drag the tab to the left/right. Then do Alt+Tab and hold Alt, so the Windows overlay stays visible. Now release the left mouse button and then release Alt (effectively you did not switch to another window). You can now drag the tab to the left and right without any mouse button pressed.
Using Windows, don’t know if it can be reproduced on Mac/Linux.