Using version 0.8.5, latest at the time of this writing, along with Firefox 3.0.3.
First of all, after I got it installed and working, it has a serious bug. On some sites, with just having it enabled (not using it), it will freeze FF when using the Back button or switching from one site to another. Not all the time, but just on some sites. If you attempt to click on the Home button in FF after this freeze, FF will crash. This happens on one of my sites, soldiersofvalor.org. Try going there with It's All Text enabled in FF 3.0.3, then go to one of your other bookmarks. Snap, it freezes as if loading the page forever. Disable this add-on and test. No issues.
Fails from the start. This add-on reports that it's unable to launch my text editor. I've tried Notepad++, Visual Studio, Notepad, and Wordpad. I'm running Win XP SP-3 with Firefox 3.0.1 and this add-on doesn't work. Very little troubleshooting or information about it from the author's site. Don't bother using if you are running FF3 and WinXP until a version is posted that is greater than 0.8.5.
Even then, with this track record of my experience, I'll never touch this add-on again unless I want a troubleshooting headache. Concept is great, execution is horrid.