Alpheios is a free add-on for Firefox that provides some Perseus-like tools for reading Latin or Greek or Arabic. “The tools will work with ANY web page or other text that is in valid Unicoded HTML, including texts you create yourself,” according to the creator’s web site (http://alpheios.net/). The tools have a help page and a video demo.
When Alpheios is installed, double-clicking on a word spawns a pop-up that has a brief definition and morphological information. Tabs on the pop-up can lead to more extensive definitions, an inflection table, pronunciation of the word (if provided) and marking the word as known. Choices on the Alpheios toolbar include access to “Greek Grammar” by Herbert Weir Smyth (revised by Gordon Messing).
So far both the Greek and Latin modules have worked well for me.