There are some extensions that change Firefox's User Agent string to
mimic that of other browsers such as IE or Opera, or later versions of
the browser.
For example, this is Firefox 1.0.7's expected user
agent string: "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US;
rv:1.7.12) Gecko/20050915 Firefox/1.0.7" and SmarterMail uses User
Agents to identify the browser and work around some of the older
browsers limitations. Altering the user agent will not allow
SmarterMail to do that.
However, I was able to
reproduce the problem after a bit of tinkering. It is a problem with
Firefox 1.0.x where it tries to turn off some AJAX functionality to
work better on the older browsers. In the meantime, however, I would
suggest that your users may want to upgrade to Firefox 1.5 or 2.0 if
possible.