Changing the default page in new tab on Firefox 3.5

"First, like Molly said, scan for rubbish.

After that, one of these two may help you:
New Tab Homepage 0.4.1 (then change the homepage to about:blank)
New Tab King 3.0.1"

"I think the best solution to provide the ability you're looking for is a Firefox add-on called NewTabURL:
NewTabURL

Select the default URL when new tabs are opened.
The options are: ""A blank page"", ""The home page"", ""The current page"" or a user defined URL.
When there is an URL in the clipboard, it is also possible to open automatically that URL in the new tab.

The two options Idigas mentioned behave a bit differently and may not have what you are looking for. New Tab Homepage loads your homepages(s) automatically in the new tab you open (which may not be the URL you want in the new tab). New Tab King opens up a menu of frequently used URL's, recently closed tabs, suggested sites. It has a lot of other doodads and configuration options such as sidebars, popup menus, etc. but I don't think you can just pick one URL for it to automatically load in a new tab."

This can be done easily. Just install the featured add-on My Homepage.