The extension doesn't work for me on Firefox v23 (ubuntu, i686-pc-linux-gnu).
The cube icon is not visible although the addon installs without an error and is correctly listed in the addons dialog.

Customizing the toolbar does not help either.

It seems others have/had this problem too:
https://github.com/blockchain/My-Wallet-Firefox-Extension/issues/1
https://addons.mozilla.org/en-US/firefox/addon/my-wallet/reviews/482352/
http://www.reddit.com/r/Bitcoin/comments/17fs03/blockchaininfo_firefox_extension_now_available/c852tq1