Help: Firefox Extension
There is a Firefox extension available to upload files from sites that have some sort of referrer or cookie access restriction. It is an alternative to the bookmarklet. The extension provides autocomplete for tags when adding a post or using the site.
Note that you need Firefox 2.0.x for the version 0.2.7 and above, which is also now compatible with the lolifox, but 0.2.6 is still available for Firefox 1.5.x users. On upgrading from Firefox 1.5 to 2.0 you should be automatically prompted to update to the latest compatible version.
As of version 0.2.8 the autocomplete function extends to the input fields on Danbooru itself.