I use Firefox, it is my preferred web browser. The first thing I do after installing Firefox is to enable the menu bar. I like the traditional pull-down menus (File, Edit, View, History, Tools and Help). This article describes how to enable the Firefox menu bar and how to maintain a solid background colour. This article applies to Windows Vista/7/8.

Update: The following procedure works with Mozilla Firefox up to and including version 28.

Update: Time has moved on... this article is now ancient history. To tame the transparency issue in Windows 7, install the latest version of Firefox and head to Mozilla's Add-ons Theme page at:

https://addons.mozilla.org/en-US/firefox/themes/

or for a history lesson, please read on...

Starting with version 4, Mozilla decided to add a transparent effect to the Firefox menu bar. The menu bar is disabled by default.
Enable the menu bar as follows:

1. Right mouse click on the open space to the right of the page tab, select 'Menu Bar' from the menu list or select the Firefox button (upper left-hand corner) --> Options --> Menu Bar

Once the menu bar is enabled the transparent effect shows up... and it looks awful!

To overcome this, install a Firefox add-on called "Firefox 3 Aero Theme for Firefox 4+ 2.0.1" written by Jeremy Morton. This returns Firefox's menu bar back to a solid colour as it appeared in Firefox 3.

However, since Firefox 16 release, this add-on no longer works. I just installed Firefox 16.0.2 (27/10/12) and the "Firefox 3 Aero Theme for Firefox 4+ 2.0.1" add-on was disabled at start-up. I discovered that Jeremy Morton has discontinued updating this theme.

After some research and a bit of tweaking I was able to reinstall the theme and it now works.

If you do not have this add-on you can install it at:
https://addons.mozilla.org/en-US/firefox/addon/firefox-3-theme-for-firefox/

Now that you have the theme installed (or was previously installed), copy the .xpi file to another folder for safe keeping. The theme is found in this location:
C:\Users\{username}\AppData\Roaming\Mozilla\Firefox\Profiles\{profileNumber.default}\extensions\
Filename: ffe_ff3aeroff4 @game-point.net.xpi

Once copied, start-up Firefox and remove the add-on (Tool --> Add-ons --> Appearance)

Before re-installing the add-on make the following modification:

Open file "ffe_ff3aeroff4 @game-point.net.xpi" (where you saved it) using 7-Zip, Winzip, WinRar or some other file compression utility.
Once the file has been opened, edit text file install.rdf.
Line 8 reads:
<em:maxVersion>16.0a1</em:maxVersion>
Change this line to:
<em:maxVersion>99.*</em:maxVersion>

Save changes and update archive file.

Alternatively, you can download the modified file here. Extract zip archive file.

Now start Firefox and manually install the modified add-on (Tools --> Add-ons --> * ˅ --> "Install Add-on From File...")
Navigate to the modified "ffe_ff3aeroff4 @game-point.net.xpi" file and select Open to install.

That's it. You should now have a Firefox menu bar with a solid background colour. Hopefully this will keep working with future Firefox releases.

These instructions also work for Thunderbird 16.01 and up if you are using the xpDeault 17.01 add-on.
Download modified "xpDefault @mikk.cz.xpi"  file here.

Note: Thunderbird v17 displays a blue menu bar. This can be returned to the normal grey colour as follows:
      - Go to Tools --> Options --> Advanced --> General and click on the Config Editor...
      - Now search for Preference Name: mail.tabs.drawInTitlebar and toggle it to false .