Last time I checked, Firefox and Safari have built-in spell checkers too (and both browsers probably had the feature before IE did). The only reason not to use Firefox is because it would cost too much to make all of the IE-only sites work in it.
Even if the apps already work on Firefox, it would cost additional time and money (creating/updating documentation, another set of security patches to track, etc) to support the new browser.
Even just testing all the apps on all the sites to find out if they are compatible with Firefox costs time and money.
