They had a free version, but yesterday took their premium version and went free with it. It has some integration with Firefox such as bookmarks and buttons. It does have e-mail integration, yet only for outlook… but that is ok.
I have checked out hundreds and this in my opinion is by far the best reader plus…
Omea Pro 2.1 replaces tools like your Email Organizer, Desktop Search Utility, RSS Reader, Personal Information Manager, Newsgroup Reader, Task Manager, Contact Manager, Bookmark Manager, and Instant Message History Manager. It can also read your Files in Microsoft Word, Microsoft Excel, Adobe Acrobat, and view your Pictures, in one Integrated Information Environment.
Oh the Firefox ext says it is only 1.5 compatibly, there is no problems with overriding max version / make compatible on install.
http://www.jetbrains.com/omea/