Yettwasezmel 1 ɣef 5 n yetran

sɛur sum1 di Aug. 23, 2009 · aseɣwen yezgan

doesn't work anymore: my Thunderbird 2.0.0.23 freezes as soon as you try to change the options of this addon.

Remember that you can change the user agent in the preferences without an extra addon:
Tools -> Options -> Advanced -> General -> Config Editor...
then in the table click New -> String and enter "general.useragent.override" without quotes and in the next input enter whatever you like. You can see that this works when you read the header of a message in your Sent folder or when you send an email to yourself.