Wondering if there is any Text to Speech software available as a plug in for IE or Firefox.
WebAnywhere is a university project aimed at creating a site that allows you to use text to speech from any browser at any location, without installing anything or using any plugins.
You can try it out and see for yourself. I was pretty impressed with it when I first heard of it.
There is also NVDA (NonVisual Desktop Access) which is an open source general purpose screen reader for Windows.
I know you asked for plugins for IE and Firefox but I thought I'd just add that Opera supports text to speech (both for issuing commands and for Opera to read web / email to you) out of the box (in Windows and possibly Mac OS).
See the Opera Voice page.
It is built in for Safari on Mac OSX. Highlight any text and go to the menu, Safari > Services > Speech > Start Speaking Text.