Current free browsers do not offer a facility to run your own modified version instead of the one delivered in the page. The effect is comparable to ‘Tivoization’, although not quite so hard to overcome," he said.
Tivoization is defined in wikipedia as a concept in which a system uses software offered under a "copyleft" license, which removes restrictions, but then uses hardware to prevent users from running modified versions. It came about as a result of an issue with Tivo digital recorders.
A programme is considered nontrivial if it defines methods and either loads an external script, is loaded as one, or makes an AJAX request, under Stallman's proposal.