Although when it was first released few months ago, some even said that this would be the end for Firefox as we know it (ha!). But as it turned out.... it's pretty popular nonetheless. I don't know if this positive feedback from the users, among their reasons but as soon as it was released they announced that, they're working on the upcoming version 5 as well!.
Mozilla has a standard procedure for their web browser release. For instance, for Firefox5 it has to pass there stages...Mozilla-central, experimental, beta and final. It has already passed the first "2 steps" now back in Friday they released the official beta version. If I remember correctly, it's scheduled to be released in June 29th.
This is not the "typical" behavior for Mozilla, but I guess that Google's Chrome release cycle may have something to do with that as well :). They've also introduced few beta channels called Nightly, Aurora and Beta which is again very similar to what Chrome has at the moment.
According to Mozilla themselves this is what these beta channels stand for...
Nightly test builds – Includes features not tested by Mozilla QA
Firefox Beta – Broadly test the stability of new features and improvements in the next version of Firefox
Firefox Final Release – Delivers the polished and stable features in Firefox to hundreds of millions of users
New version brings bug fixes and enhanced memory management and other typical things ... but most importantly, Firefox5 beta brigs dedicated CSS support to their web browser which can be used to create more "cool" looking buttons, etc on web sites which is certainly good news for web developers without a doubt.
As a GNU/Linux user, you certainly don't have to wait till June to "taste" FF5 to use it on your Linux Laptop/PC... in fact thanks to a launchpad PPA channel... you can easily install the beta version already.
So, if you want to install Firefox5 beta in Ubuntu 11.04 Natty Narwhal then simply open your Terminal and issue the below command. But remember, this is still at its beta stage... use it at your own risk :).
sudo add-apt-repository ppa:mozillateam/firefox-next
sudo apt-get update
sudo apt-get install firefox
sudo apt-get upgrade