List of my favorite Firefox 3.0.x compatible extensions:
add_bookmarks_here
adblock_plus-0.7.5.3
bugmenot-2.0
copy_link_name-1.2.6
copy_plain_text-0.3.3b
copy_url_plus-1.3.2
cutemenus_-_crystal_svg-1.9.3
dictionarysearch-3.0.0
downloads_in_tab-0.0.6
downthemall-1.0.3
extended_statusbar-1.2.8
firebug-1.2.0b3
flagfox-3.2.6
gmail_manager-0.5.5
google-notebook
gtranslate-0.5.1
jsview-2.0.4
hyperwords-4.0.2
permit_cookies-0.6.2
quoteurltext-1.0.8
right-click-link-1.1.3
tab_mix_plus-0.3.6.1
to_read_later-1.2.0
user_agent_switcher-0.6.11
video_downloadhelper-3.0.4
web_developer-1.1.6
And also some Themes...
qute-3.3.2
Noia2