<Control> + <Shift> A. There are add-ons that you may like to use at Mozilla Add-ons Web Page There's lots of good stuff here.

I'm not sure if you can open such a menu with the keyboard.

Does it open if you left or right click the mouse on such an icon on the Add-on Bar?

Yes, the add-on has a user interface which is displayed when I click on the add-on's icon in the add-on bar with the mouse. See the attached screen shot which shows the add-on bar. I wanted to know if there is a way to open it using the keyboard.

Looks like a bug accessibility bug in Firefox. How do I reports bugs about Firefox?

Screen shot

I can't figure out how to tab onto the Add-on bar, or tab to any icon in the Navigation bar. Am I missing a setting?

If you tab through the whole web page then finally you will arrive at the add-on bar and the add-on's icon gets focus but then pressing enter or space bar does not open the add-on's user interface.

Hi RamDosNT, I've never been able to replicate this, I can never get the cursor to move onto my Add-on bar when I tab through the cycle (address bar, search bar, body, links and form controls in the document, up to tab bar, back to address bar).