Firefox can be customised in many ways using the about:config URL. But sometimes, you may mistakenly enter a value or you may want to reverse an entry completely.

However, if you've ever tried to delete a custom entry, you'll know there's no option labelled "delete". It's actually the "Reset" option you want to choose if you want to remove a custom entry. Here's how it's done:

1. In your Firefox address bar, type about:config and hit enter. You'll see a standard warning about making changes to the config. Accept this to continue.
2. Now, you need to find the entry you want to delete. Type a part of the entry name, and the list of settings should filter to reveal the one you want. Custom settings are usually highlighted in bold type.
3. All you need to do now is right-click on the entry you want to delete and select Reset. If the setting is part of the core browser, it will be reset to the default value. However, if it was a setting you added, it will be deleted.
4. Restart Firefox in order for the changes to take effect.
5. If you like, you can check the changes by revisiting about:config.

Hope this helps a few people to get rid of old or troublesome customisations in Firefox. It's a lot more straightforward than resetting your profile!