One possibility is
@-moz-document regexp("^a?!(bout:).*|^[^a].*")
However, testing your regex, it works for me in any case. Are you enclosing the rest of the script in braces afterwards?
@-moz-document regexp("(^(?=(?!about))){1}.*") {
/* rest of script */
}
If so, you might have the wrong idea about what the about:
url is matching. This only matches the page contents of about:
pages. Note that any Firefox UI elements that are affected by your style will still be affected (even when you're visiting an about:
page).
As an aside, it's convenient to use a simple userstyle like this for testing, so your script's behaviour won't complicate things when testing the regex.