There is no Firefox 3.9. Did you mean Firefox 3.6?

What don't you like about Firefox 4?

You can customize it to make it more like and behave more like Firefox 3.6 if you want, for details see http://redherring2.wordpress.com/2011/01/17/firefox-4-restoring-firefox-3-ui/

I don't see any benefits to Firefox 4, either. All of my add-ons are no longer valid. The colors are bland and look like Internet Explorer. I checked for a persona and nearly all the choices are grey out as not valid for Firefox 4.

I would like to return to the previous version, too.

If you would still like to downgrade to the previous version, you can download Firefox 3.6 from this page. Please be advised that support for Firefox 3.6 will continue only until about mid-2011. Once support for it has stopped, it may become more vulnerable to security flaws. At that time, it is highly recommended that you upgrade to Firefox 4.

You can check the developers site of the add-ons to see if they have made updates for Firefox 4.

You can change the theme, for example https://addons.mozilla.org/firefox/addon/firefox-3-theme-for-firefox

To downgrade to Firefox 3.6.* you can uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install Firefox 3.6.* available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

don't like firefox 4, too much empty (wasted) space at top. Moving things around has not made anything more convenient. Seems that folks can never leave well enough alone; however, I do realize that I had the option of not updating.

Perhaps I received a corrupted version. There was not a tool bar (File thru Help) and therefore unable to work with it. No View option so bookmark listing wouldn't appear on the side, and some of my bookmarks would open only marginally - not full screen. At least 3.6 was easily reloaded.

this version, 4.0, is incompatible with most of the applications of the old version. so, we need the old version or a debgging of this problem, because every on people who instal an application need this. And the oldest versions are more light.

downgraded back to 3.6, I can't stand the 4.0 setup, it is too different and personally the 3.6 interface is much nicer to use. 4.0 is just too empty, as stated above it looks too much like IE