Most likely, Firefox 4 is using a different style of color management that is making your images look different.
If it's bothering you, you might want to take a look at the Color Management add-on - with this, you can set the color management to no images, all images, or just images with embedded ICC tags.
For a brief explanation of color correction in Firefox, take a look at ICC color correction in Firefox. It refers to the change from Firefox 2 to 3, but it shows how color management profiles can affect image color.
Although support for color correction was introduced in Firefox 3, it was disabled by default, requiring some tinkering about in the about:config window to enable it. Firefox 3.5 addresses the issues that caused it be disabled by default in the previous version, and now images with International Color Consortium (ICC) tagging are, by default, color corrected.
The image below is split into three sections. The top left corner shows the image as rendered by Firefox 2. The top right corner shows how the image renders in Firefox 3. The bottom shows the image rendered in Photoshop.
As you can see, Firefox 3 and Photoshop render the image identically, because both support the embedded color correction profile. Firefox 2 ignores the profile, resulting in mismatched color.