List of my favorite Firefox 2.0.x compatible extensions:
adblock_plus-0.7.2.1
addbookmarkshere_0_5_6
bookmarks_linkchecker-0.6.8.3
bookmarksSynchronizer-1.03
bugmenot_1.3
copyplaintext_0.3.2
copy_url_plus-1.3.2
cutemenus_-_crystal_svg-1.9.1
dictionarysearch_2.0
downloads_in_tab-0.0.6-fx
downthemall_-0.9.9.7
extended_statusbar-1.2.8
firebug-1.05
flagfox-3.2.6
gmail_manager053-en-US
gmail_manager053
gtranslate-0.5.1
HTML_validator_linux_0839
hyperwords-4.0.2
ietab-1.2.0
ieview_1.3.1
jsview-1.2.1
locate_in_bookmark_folders-0.2.5
menu_editor-1.2.3
permitcookie_0_6_2
popupsound_0.1.5
readitlater_0.8
right-click-link-1.1.3
statusbarClock_FF20
tab_mix_plus-0.3.6
textlink_0.3
tinyurl_creator-1.0.2
titleBar_FF20
user_agent_switcher-0.6.9
video_downloadhelper-3.0.4
view_source_chart-2.5.03-fx
web_developer-1.1.3
And also some Themes...
azerty_iii-3.1-fx
noia_2.0_extreme_-3.30-fx
noia_2.0__lite_-3.21-fx
qute-3.2.2-fx