Rated 4 out of 5 stars

My Firefox 4 work fine with InfoRSS 1.3.0.
If you update to FF4 from FF 3.x.x, maybe remove "previous" InfoRSS in the Extensions page first, restart your Firefox 4, and back here re-install "new" InfoRSS. Afterwards, customize the Toolbar to show "Add-on Bar" on FF4 UIs.

This review is for a previous version of the add-on (1.3.0.1-signed.1-signed.1-signed).