ok, I've got it! (after searching a lot, it was very simple...)
you just need to this:
Try (re)moving extensions.rdf, extensions.ini, and extensions.cache from your profile (exit Firefox first). These will be regenerated on the next run, but will cause any previously disabled extensions to be reenabled.
The profile can be found inside you user folder/application support/Firefox then browse for the profile folder and inside, there will be an extensions folder.
Then, drag the new extensions into the extensions folder in your profile.
It's incredible the way the new extensions improve it!!!
I've tested the following: (you can download them from the mozilla site)
add_n_edit_cookies-0.2.1.0-fx+mz.xpi
all-in-one_sidebar-0.6.4-fx+fl.xpi
cache_status-0.6.0-fx.xpi
codetch-0.3.6.20051230-fx+mz+tb.xpi
download_manager_tweak-0.7.1-fx.xpi
download_statusbar-0.9.4-fx.xpi
downthemall_-0.9.8.7-fx.xpi
editcss-0.3.4-fx.xpi
image_zoom-0.2.3-fx+fl+mz+ns+tb.xpi
linkchecker-0.4.5-fx.xpi
measureit-0.3.5-fx.xpi
public_ip-0.3-fx.xpi
server_spy-0.1.0-fx.xpi
testgen4web-0.38.1-fx.xpi
view_formatted_source_format_source_extension_-0.9.4.2-fx.xpi
web_developer-1.0.2-fx+fl.xpi
window_resizer-0.5-fx.xpi