The most notable change in this version is the increment in version numbers from 3.1b3 to 3.5b4. As noted in my previous post this is due to the inclusion of a number of significant bug fixes and additional features to the code base.
This latest beta includes updates to TraceMonkey, Firefox’s  Private Browsing Mode and also support for more CSS and  HTML5  attributes. You can read a full list of updates in the beta release notes.

For those interested in downloading and testing this new version, here is the beta release email.

Greetings to all of our great community testers – you are our most
valuable resource. We now have Firefox 3.5 Beta 4 available for testing
(Note: Since my last missive, we have changed the release name to
Firefox 3.5, the release formerly known as Firefox 3.1). Note that this
build is a beta and is intended for pre-release testing. We would like
your help in testing the release and identifying any possible
regressions or issues.

*What to Test/Where to Get Builds**:*

(Note: If you are riding the “beta” channel, you can manually check for
updates and update to this build). Otherwise you will have to download
from the directory below.

http://www.mozilla.com/en-US/firefox/all-beta.html

Please hammer on these builds mercilessly to make sure that things work
well! If you notice things that worked in Firefox 3.1 Beta 3 and do not
work in this release, we would like to know about it right away.

Read the Release Notes
*Please also make sure to read the Release Notes which detail the known
issues: http://en-us.www.mozilla.com/en-US/firefox/3.5b4/releasenotes/

***_How to Report Feedback_:*
(1) File a bug in *https://bugzilla.mozilla.org/. *Note in the comment
if it is a regression from a previous release. Things to remember when
filing a bug:
(a) Always include the Build ID that you tested on. If you type
about: in the URL bar, this will give you the Build ID.
(b) Always include clear Steps to Reproduce the bug
(c) Always check to see if your bug has already been filed. This link
will help: http://tinyurl.com/2465be
(d) Use the regression keyword if it indeed a regression from a
previous release.
(e) If you happen to crash, please include the Breakpad ID in the bug.
You can get this by typing about:crashes in the URL bar.

(2) If you don’t wish to file a bug, report issues through
http://feedback.mozilla.org or through the
mozilla.feedback.firefox.prerelease newsgroup (also available on Google
Groups). However, we prefer bugs as feedback is easier to track.

Thanks in advance to all of you for helping test Firefox and making it
the browser of choice for millions of people all over the world! We are
getting ever closer to the final release and thank you for your
participation.

marcia, on behalf of
Team Mozilla QA

Mozilla have announced that the forth beta version (due for release 14th April) of the next instalment of popular browser Firefox will skip a few versions from 3.1 to 3.5.

Firefox director Mike Beltzner announced on the developer news blog that due to the inclusion of many new features (TraceMonkey, Private Browsing , etc not to mention the number of bug fixes already added to Firefox 3.1 ment that an increase in version number to reflect the changes was required.

Further updates for us Add-On developers is to be announced in due course via the wiki, and probably the mailing list too.

for more information, visit these sites:

• mozilla developer news
• mozilla developer wiki
• the register