MacType - MacType render windows font smoothly and much more convenient to read.

Swift Selection Search - Swiftly access your search engines in a popup panel when you select text in a webpage.

gdipp - The gdipp project is a replacement of the Windows text render, which brings to you the effect of...

Trufflepiggy - Quick Search - The all-in-one search addon: just select and search. Find on multiple websites at once. Search via overlay, context menu or omnibar. Featuring anonymous profiles. Easy setup: select from 1000+ user generated searches. Cross-browser sync.

fontconfig - Fontconfig is a library for configuring and customizing font access. Fontconfig can:

Selection Search - With this extension you can easily search for text you have selected.