Description of problem: Since it's not usefull to run two different firefox versions on the same system it makes IHMO no sense to have the version number of the latest installed firefox version in the /usr/lib/ directory of firefox. Another reason is that users which place there symlinks for firefox plugins like java in /usr/lib/firefox x.y.z. have to modify these symlinks after every firefox update.
If you want to install plugins system-wide, they go in /usr/lib/mozilla/plugins If you want do not have write access to this directory, or want to install plugins for your user only, they go in $HOME/.firefox/plugins