All have versions available that are compatible with Firefox 1.5...
- Adblock Plus - block adverts
- DOM Inspector (built-in) - explore DOM trees
- downTHEMall! - batch download files
- LiveHTTPHeaders - shows HTTP headers, good for debugging
- LiveLines - configure the destination for feed subscriptions
- MIME Type Editor - set default behaviours for file types (for OS X, 'Open with /System/Library/CoreServices/BOMArchiveHelper.app' often works well)
- Piggy Bank - store and present semantic data
- SessionSaver - store collections of pages and recover from crashes
- Slogger - log and archive visited web pages
- Stylish - apply CSS styles to specified pages
- Tab X - add a close button to each tab
- Web Developer - huge amount of tools for web development
And honourable mentions to BugMeNot, Platypus and User Agent Switcher, which come in handy occasionally.