From the developer's own blog: New Bookmarks and History, Tabbed Browsing Enhancements, Improved Basic Content Type Handling, Web Search, Visual Uplift, Inline Spell Check and more.
New Bookmarks and History
Improve the browser's Bookmarks and History systems to improve their effectiveness as renavigation aids while at the same time improving the back end for speed and extensibility.
Tabbed Browsing Enhancements
Make tabs behave more like windows in the operating system environment, making them behave more as users would expect.
Improved Basic Content Type Handling
For things like RSS/Atom feeds, mail links etc. Improve discovery and handling user interfaces.
Improve the discoverability and adaptability of the search UI within Firefox.
Bug fixing at all levels where risk is low and yield high, e.g. the blank tab download bug, platform stability, etc.
Some additional things we would like to look at in the Firefox 2 timeframe include:
A freshen of the visual design of Firefox while maintaining high levels of system integration.
Inline Spell Check
The rise of applications like web mail, blogging etc highlight the weaknesses of HTML's textarea widget. We should at the very least offer people the ability to spell check their submissions.
We'd like to know why people leave Firefox. A survey on uninstall would help us find ways to make the software better in future versions.
UI Consolidation and Simplification
Consolidate and simplify user interface in the browser window tying together features in meaningful ways where possible.