These add-ons caught my eye as being useful... but I do not have use for them yet so I have not loaded or tested them. Still, they are worth remembering.
Adblock Edge is a fork of the Adblock Plus version 2.1.2 extension for blocking advertisements on the web, without sponsored ads whitelist.
**This addon will be discontinued on June 2015**
Schützt Ihre Privatsphäre durch das Blocken von Tracking
Auto Refresh will refresh or hard refresh any tab or group of tabs at individual or group intervals.
You can also customize your presets and save the ones you most use.
Integrates location bar and search bar into one. Shows search and url suggestions. Quickly switch search engine using search keywords. Provides different auto-complete popup styles for compact displays.
Change the case of any selected text in just one click from the context menu. Change case to lower case, Sentence case, Capitalise Each Word, UPPER CASE and tOGGLE cASE.
Quick lookup in Wiktionary and Wikipedia, without the need to open a new tab.
Just right-click a word while shift / ctrl / alt is pressed.
A full-featured scientific calculator which can be placed in any Firefox tool-bar or the Australis PanelUI.
Install it --> Right click on any image when you on Google Plus --> Click on "Full-size" item.
This will open a full-size image in a new tab. Works with any image in your stream (but not for the external links), thumbs and small profile images.