Wondering if there is any Text to Speech software available as a plug in for IE or Firefox.
It is built in for Safari on Mac OSX. Highlight any text and go to the menu, Safari > Services > Speech > Start Speaking Text.
For Firefox, there is LowBrowse, which has text to speech for the paragraph that the cursor is in.
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.
Windows Narrator is part of a default XP installation, and can be used in any browser. I also suggest you try out Thunder, which is free and can work with IE and most other Windows apps.
There is also NVDA (NonVisual Desktop Access) which is an open source general purpose screen reader for Windows.