This is partially a bug in firefox-3.0 also: it puts files directly in common directories (/usr/lib/firefox-addons) and symlinks in the version-specific directories (/usr/lib/firefox-3.*). So if the firefox-3.5 were to provide those files also, they would conflict with firefox-3.0 files.
To make the two packages peacefully coexist without being mutually dependent, the situation should be reversed: install the search plugins in the version-specific directories, and if necessary, use the alternatives mechanism to manage a symlink from the common directory.
The same situation applies for the /usr/bin/firefox (no version) symlink.
(I've also resolved the issue on my system using an approach like comment #15 (by Bubbels).)
This is partially a bug in firefox-3.0 also: it puts files directly in common directories (/usr/lib/ firefox- addons) and symlinks in the version-specific directories (/usr/lib/ firefox- 3.*). So if the firefox-3.5 were to provide those files also, they would conflict with firefox-3.0 files.
To make the two packages peacefully coexist without being mutually dependent, the situation should be reversed: install the search plugins in the version-specific directories, and if necessary, use the alternatives mechanism to manage a symlink from the common directory.
The same situation applies for the /usr/bin/firefox (no version) symlink.
(I've also resolved the issue on my system using an approach like comment #15 (by Bubbels).)