Answer by Michael Paulukonis for Javascript bookmark stopped working in Firefox 13

NOTE: this solution no longer appears to work as of FF41. See JS Bookmarklets stopped working in Firefox 41.

If you first load almost any URL, including about:blank, then a Javascript bookmark will work in that tab.

Since Firefox's default behavior for new tabs is about:newtab, which is nothing, and bookmarklets only run once something is loaded, you can do the following to set a default page, and then run bookmarklets:

1. open about:config
2. find browser.newtab.url
3. double-click and change from about:newtab to about:blank (or URI of your choice)
4. Ctrl-T and run bookmarklets in new tabs!