List of my favorite Firefox 1.5.x compatible extensions:
adblock_plus-0.7.1.2.xpi
addbookmarkshere_FF1.5.xpi
bookmarks_linkchecker-0.6.8.3.xpi
bookmarksync-102-FF1501.xpi
bugmenot_1.3.xpi
chatzilla-0.9.70-fx+mz.xpi
contexthighlight_0.2.xpi
copyplaintext_0.3.2.xpi
copy_url_plus-1.3.2.xpi
cutemenus2-1.0-FF1.5.xpi
dictionarysearch-1.5.xpi
downthemall_-0.9.9.7.xpi
gmail_manager053-en-US.xpi
gmail_manager053.xpi
gmail_notifier-0.5.6.2.xpi
HTML_validator_linux_0839.xpi
ietab-1.1.1.4.xpi
ieview_1.3.1.xpi
ie_view_lite-1.1.5-fx.xpi
jsview-1.1.7.xpi
locate_in_bookmark_folders-0.2.5.xpi
permitcookie_0.6.2.xpi
plain_text_link.xpi
popupsound-0.1.4-fx.xpi
sessionsaver_FF1.5_compatible.xpi
statusbarClock_FF15.xpi
tab_mix_plus-0.3.0.5-fx.xpi
tinyurl_creator-1.0.1.xpi
titleBar_FF15.xpi
undoclosetab.xpi
user_agent_switcher-0.6.9.xpi
venkman-0.9.85_FF1.5.xpi
And also some Themes...
noia_2.0_lite_-2.996-fx.jar
noia_2.0_extreme_-2.996-fx.jar