Ooh, I'm excited with this thread. Being new to FF, I can learn some useful shortcuts here and share my 'by mistake' shortcuts I found.
Example:
1. Typing '.' in the highlighted URL bar brings up a very useful list of tabs (but I haven't figured out exactly what that list is yet. All I know is that I use it all the time since I have so many tabs open.)
2. For others who keep open so many tabs also, there is a down arrow at the very right of the tabs (to the right of the '+') which opens up a menu of all of your tabs.
3. At the very bottom of that drop down menu is an eensy weensy down arrow to scroll down to more tabs that don't fit on the menu. (or an up arrow at the top of the menu if you want to go back up the list of tabs)