Comment 4 for bug 865302

Confirmed that this same behavior occures in 12.04 LTS. The following behavior has been observed:

After login, open thunderbird. After some period of usage, thunderbird will no longer appear in the alt-tab window switcher. In order to get the icon to re-appear you must run "unity --replace", as stated above. Since having found the above work around I've yet to have the icon disappear on me again (roughtly 30 minutes). If I can come up with any repoducable steps I will post and update.