There are more than one ways to hide User Interface (UI) components from Firefox. The most straight forward is via the userChrome.css file that firefox respects, by default. It is sufficient under the firefox profile directory to create a directory named “chrome” (if it’s not already there) and then write your own userChrome.css file. This file will allow firefox to apply all the modifications you specified and therefore, change the interface according to your needs.
An example file here: userChrome.css (Sample File)
Drawbacks: The userChrome.css file could contain ids, class names, labels, etc…. Inconsistency on these item names between different firefox versions will result to non-fully-functional customizations.
Advantage: You do not have to install any separate firefox add-on (e.g. MenuEditor).
Tools: Build your userChrome.css with DOM Inspector.
Opening the following line with DOM Inspector: