There is a known issue for some i3 users that when we click the right-mouse button, the context menu will pop up in a position where the first item in the menu is already highlighted, so when the right mouse button is released, it automatically "selects" that item. So if we want to copy a link, for example, we have to be careful to move the mouse before releasing the button or that link will immediately be opened in a new tab. What some of us have done in the past is to create a userChrome.css file setting a top (and/or left) margin for that menu, which fixes the issue. In the latest version(s?) of Firefox, this no longer works (but other userChrome.css rules I have set up continue to work), so I'm assuming that the ID of the context menus has been changed. What is the ID now?