100 Amazing Firefox Add-ons for Homeschoolers, OnlineDegrees.net (April 2010)
This list of tools was compiled for homeschoolers, but there is gold here for everyone. All are add-ons for Firefox but you can get the equivalent of most of these for Internet Explorer, and can also use the resource sites directly.
Has categories for:
+ Math
+ Science
+ English Language Arts and Reference Materials
+ Foreign Languages
+ Productivity
+ Research (mainly tools for saving, adding notes, and sharing)
+ Parental Controls and Computer Security
+ Search Engines and Web Browsing
+ Miscellaneous
To this I would add tools for access books online, especially the Internet Archive. And - as it happens - there is one for the Internet Archive. "Search for anything on the archive and it gets sorted by popularity/most downloads. "
You may find others. Run a search at Google for firefox (plugin OR addon)