Everyone has their own preference for what elements 'tab' should cycle through. Personally, I like it to cycle through everything, but I can certainly understand those who just want it to cycle through form fields. But only text fields (the default in OS X, at least)? Come on, what's the point? : )
Firefox has a hidden preference called 'accessibility.tabfocus' that controls what 'tab' cycles through. It can be set to any of the following values:
- 1: only text fields
- 3: all form elements
- 4: only links and linked images
- 5: text fields, links, and linked images (but not other form elements) (1 + 4)
- 7: all form elements and links (3 + 4)
Really it seems like 3 and 7 are what most people would find useful.
Setting An Option
In Firefox, type 'about:config' in the location bar (or whatever it's called) to bring up the 'hidden preferences'. Use the filter at the top to search for 'accessibility.tabfocus'.
If it exists, set your preferred value by double-clicking on it.
If it doesn't exist, you'll have to create it. Right-click somewhere in the field of preferences, and select 'New > Integer'. Enter the preference name as 'accessibility.tabfocus' and the value as whatever value you'd like.