Firefox displays the close button next to each tab by default in the most recent version. What this means is that you see an x icon next to each open tab in the browser.
While that may be useful to some users, others may dislike it as it takes up a lot of space in the tabbar.
Up until now, Firefox users could modify the browser.tabs.closeButtons preference in about:config to modify where the close button appears.
Options included displaying the close button on the active tab only, at the end of the tabbar, or not at all. If you have selected the last option, you would have to close tabs using the right-click context menu, the shortcut Ctrl-W, or add-ons that provide that functionality.
My personal preference has always been to display the close button at the end of the tabbar. Not only do I save space as it is only displayed once, I can also use it to quickly close multiple tabs without having to move the mouse button.
As of Firefox 31, the preference has been removed from Firefox. The reasoning behind the decision is explained by Mozilla employee Jared Wein:
The browser.tabs.closeButtons pref is only discoverable via about:config. It provides other locations for the tab close buttons, but none that we plan to move towards in the future.
These would be better supported and more discoverable if they were implemented by an add-on.
Because we are removing this pref, we will effectively be migrating users back to the default value of 1.
Not only is the preference removed, all users who have set up a custom value will notice that the value has been reset on their systems so that the close button is displayed on all tabs in the browser.
How to fix it
The only option that you have is to install a browser extension to restore the functionality. Aris, the developer of Classic Theme Restorer has integrated the functionality in the latest beta version of the extension (check out our full review of it here).
You find the tab close preference on the Main tab of the extension. Here you can set all four different tab close modes again by selecting the one you want used from the menu.
The removal of the preference broke several add-ons that made use of it. While some of this may be resolved in the days and weeks to come, add-ons that are no longer maintained will be partially broken as a consequence.
Removing features like this, that have been part of Firefox for a very long time, will only affect long-time Firefox users who have been using the browser for years. Most new users probably do not even know that there is an option to modify the tab close behavior in the browser which means that they won't be affected by the change at all.
Where do you prefer the tab close button to be displayed in Firefox?