hello, please note that not all desktop addons are available on firefox for android. here is a list of compatible ones: https://addons.mozilla.org/android

Thanks for your suggestion. Followed the link but the problem remains with all Add Ons as per my original post.

did you use firefox on android to post the original question too, because the useragent submitted (Mozilla/5.0 (Linux; U; Android 4.0.4; en-us; Xoom Build/IMM76) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Safari/534.30) looks more like the default browser... or do you already use an addon that changes the user-agent?

I used FF for Android on Sony Tab S for question then replied FF for Android from SGS2. Yes I have user agent Changer app already. It is only recently I have not been able to install Add ons.

please try to disable the user-agent changer for the moment or have it identify you as firefox mobile to install a new extension...

Disabled Phony user agent changer after trying various user agents including default - all the same. If I refresh the page the install bar flashes green momentarily then reverts to white and it is unusable.

can you enter about:config into the address bar, search for all preferences starting with general.useragent and make sure that they are on their default state (otherwise a reset button will appear when you select them)?

Great! general.useragent.compatMode.firefox was "true". Reset button appeared. Reset to false, restarted FF and hey presto was able to install add on! Thanks :-)