Firefox - is a very powerful browser with lots of features. Here's a list of some important and useful features.
In the bookmark Right Click Properties menu, Keyword field is for short aliases.
For example, for a Facebook bookmark, set keyword as fb. Next just type fb in address bar and hit enter and it'll open up Facebook.
When opening bookmarked sites frequently, this feature comes very handy.
Creating Tab Groups is again a very useful feature in Firefox. Ctrl + Shift + E or Clicking the Tab Group Icon on the menu bar opens up the Tab Group View. Similar tabs can be groups by drag and drop and groups can be named.
Ctrl + ` shows up Next Tab Group and Ctrl + Shift + ` shows the Previos Tab Group.
Pinned Tabs are viewable across all tab groups.
Pressing F7 toggles between caret browsing. This feature when enabled, allows to navigate the web page like any word processors - using Arrow keys to navigate. When enabled, its places a cursor in the webpage which can be moved around using arrow keys. Shift + Arrow Keys for seleting any text.
To open a link in new tab in background, press mouse wheel over the link. Useful like while opening many search results from the Google search page etc.
Some very useful keyboard shorcuts
\ : Quick search within the page.
Address auto complete.
Many of us normally use the Ctrl + Enter to add .com in the address. But it is helpful with one more nice option. For example, to open Google reader (http://www.google.com/reader) simply type google/reader in the address bar and hit Ctrl + Enter and it will take to Google Reader site.