Rated 4 out of 5 stars
Great add-on, however I can suggest two improvements:
1) when using "about:config" (rare, but it happens - I myself must add some general.useragent.override item in order for FF to work with my provider (SFR - Vodaphone france)), the fonts are also doubled by Bigger Text, and the preferences are then quite hard to read/modify (the text is too large); since this page is not scalable by pinch/zoom, the only workaround I found was to disable Bigger Text and then re-launch FF, just to access "about:config".
2) After having downloaded a (normal) webpage with 200% Bigger Text, if I zoom/pinch, it is then not possible to come back to the initial 200% rendering (zoom by double tap switch between the normal 100% view and the zoomed page). It would be nice to have a way to come back to the 200% view without having to reload the page.