I've used firefox and .rtf for years and never have problems*. (In fact I highly recommend .rtf in general, as it's not nearly as annoying as .doc can get.)

* Though after I insert a horizontal line break between my header and my title (which I usually have both bolded and underlined as well as centered), it does strip the underline from the title. Other than that though, it keeps everything. (And it does keep the underline until I insert the linebreak.)