One of the interesting things is that Mozilla's Fennec browser will have extensive support for add-ons:
Alpha 2 has made significant improvements to Fennec's performance (e.g., Faster panning and zooming plus improved responsiveness while pages are loading) and ease-of-use (e.g., Bookmarks, tabbed browsing with thumbnails, etc.)
But Ars Technica picks up on one of the best new "features" of Fennec:
As Fennec development continues to move forward, the value and significance of having the complete Firefox stack in a mobile environment is becoming increasingly apparent. Developers have already started creating innovative add-ons for the new browser that increase its functionality in various ways. For example, the TwitterBar extension allows users to post to Twitter directly from the Fennec address bar. An early Fennec port of Mozilla's Weave framework is also underway.
Like Apple's iPhone rendition of Safari, Fennec may well prove to be most disruptive when replicating and extending the desktop experience in a mobile device. This is where open-source Fennec could leapfrog its proprietary competition, including the iPhone's Safari.