Matthias Schoettle

Lead Software Developer

Category: Firefox

Digging into the Firefox UI for the tab close button

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.

Firefox close tab button on hover

There are probably extensions that allow to do that, however, this is not necessary as I will show in this post. Maybe you’ve seen the functionality in Safari or just wondered why the close button for tabs in Firefox can’t just always be there. In Safari, the close button appears when hovering over the tab itself.

The following modification adds this functionality to Firefox. You need to create a file called userChrome.css with the following content:

.tabbrowser-tab:not([selected]):not([pinned]) .tab-close-button {
        visibility: hidden !important;
        margin-left: -16px !important;
}

.tabbrowser-tab:not([selected]):not([pinned]):hover .tab-close-button {
        visibility: visible !important; 
        margin-left: 0px !important;
        display: -moz-box !important;
}

Place this file into the chrome folder inside your profile’s directory. Follow the directions from Mozilla to find out where to find the profile’s directory location.

Update 1: It just tried this again myself due to a fresh installation and you also need to change the browser.tabs.tabClipWidth preference from it’s default value (140) to 99.

Update 2 (August 2017): As pointed out by Veto in the comments below, this functionality is broken since version 55.0.3. The above CSS has been updated to stay compatible with the new Firefox UI.

Update 3 (December 2017): As pointed out in the comments, pinned tabs were affected too. Thanks to Mike for the solution on how this can be avoided. The above CSS has been adjusted.

Then just restart Firefox and you are done.

Original Source: Post on Neowin Forums

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