Firefox is a great cross-platform browser. It competes with Safari for rigid standards compliance and is relatively quick and solid. It falls down a bit on the UI. One of the more frustrating problems is the continued inconsistency with middle mousebutton clicks. Some builds support clicking the middle mousebutton to open a page, and others do not.

The menus also aren't fully integrated so you can't configure shortcuts for some actions, and many of the more useful configurable options are hidden in a cryptic about:config page, and while the program is otherwise fairly stable, it leaks memory and can eventually bring the system to a crawl if you don't periodically exit and reload.

Where Firefox excels is in its plugin support. Like Safari, Firefox has a versatile plugin architecture, but unlike Safari there are literally hundreds of plugins to choose from. The usual candidates are there - advanced advertising filtration and newsreader enhancements - but you can also find web development plugins. GreaseMonkey is one such plugin, and it's highly configurable and takes plugin scripts itself -- it makes automating web tasks and finding problems with page layouts a breeze. There are also plugins for automatically checking HTML/XML validation of the current page, and there are even site-specific plugins adding great features to Netflix and many discussion boards.

On top of all this, you can't beat the price -- Free.