Well the best and easiest is to use distribution packages but … some distribution have slow package updates so you need to do it your self. Here is short text how to do it.

1. switch to super user (root) account
2. Download firefox - be careful - choose right versions for your system (32-bit | 64-bit)
3. Unzip and untar file to some dir. So you will than have sub dir firefox
4. mv firefox dir to /opt
5. create new links so that correct firefox is started.
6. create new links in for plugins in '/opt/firefox/plugins' so that you have usual plugins flash and java.
7. future upgrade after above installation procedure

The procedures from 1-5 are well documented here.

Creating new links for plugins

Go to new firefox dir

>cd /opt/firefox

If plugins dir does not exist do:

>mkdir plugins

go to plugins dir

>cd plugins

Create symbolic links to plugin libraries you want to use. This depends where are plugins located. From my experience it is better to use plugins which come with distribution but you can download plugins from any source (you can than expect dependency problems). In my case (OpenSUSE) plugin links are created like (you must be in /opt/firefox/plugins directory)

>ln -s /usr/lib/browser-plugins/javaplugin.so
>ln -s /usr/lib/browser-plugins/libflashplayer.so

By default when new firefox version is ready firefox will try to update automatically. If you are regular user it won't work (update dialog will stuck downloading update) as you do not have needed file permissions.

The easiest way if to start firefox as root and go to Help→About. There are all options for update (usually update is started automatically) but you must be root for update to work.

Harder way is to repeat above installation procedure for new version.

1. Just rename /opt/firefox to /opt/firefox_old
2. do new version installation as described above
3. if everything is working correctly after upgrade delete /opt/firefox_old