Oops … last year a new Firefox version broke the “close button on tab hover” tweak and I mentioned I will write a separate blog post on how I found out why and how to fix it. Life happened, but fortunately I wrote down bullet points about what I did.
It seems that with Firefox 54 something in the UI changed which broke the functionality. Fortunately, it wasn’t the removal of the ability to customize the UI using
userChrome.css. In order to find out why the CSS selector didn’t work anymore, it was necessary to see how a tab was structured in XUL.
A while ago I used DOM Inspector. The current Firefox version then was 57, but it didn’t allow legacy extensions anymore. So I went back to the last 54.x version, which allowed the installation of this “legacy” extension. Unfortunately, I could not open its UI. I did however find the extension InspectorWidget. It has a nifty shortcut to open the inspector for the desired element right away. Hold Ctrl+Shift while clicking on the desired UI element.
Using the inspector I found out that there was a new CSS rule that set
display: none for certain elements, one of them being the close button for tabs. To find out the initial/default value I used the computed rules, which showed
-moz-box. This allowed to add
display: -moz-box !important to the rule for hovered tabs to make it work again.
While it doesn’t seem possible to inspect the UI anymore with Firefox Quantum due to the introduction of WebExtensions, the CSS rules still work. However, it is unsure whether these kind of UI customizations will stay. There seemed to be plans to remove it., but perhaps they realized how many users are using this to tweak their UI since extensions are not allowed to do it anymore. This bug report to collect usage of
userChrome.css supports this theory.
Update (2023): In the last few years there has been a much easier (built-in) way to inspect the Firefox UI using the Browser Toolbox. See this post on Super User.