I don't think it's as much of a "firewall" in the sense traditional news media (theoretically) tries to maintain as much as a fairly natural separation between the kinds of companies advertising with him and the kinds of topics he writes about. In practice, the chances are fairly small that he's going to have a reason to write negative things about Squarespace, let alone Desk, assuming he's done basic vetting of the product/service.
The failure mode for this theory, of course, is if one of the companies -- especially a service like Squarespace -- either shifts their business model to something slimy or is revealed to have been doing something slimy all along. Personally, I don't think Gruber would hesitate to both report on that and stop taking their ads -- doing so would have little to no effect on his bottom line.
(The observation made by another commenter about how many bloggers and, of course, podcasters pump Squarespace without actually using Squarespace is wryly amusing, though.)
The failure mode for this theory, of course, is if one of the companies -- especially a service like Squarespace -- either shifts their business model to something slimy or is revealed to have been doing something slimy all along. Personally, I don't think Gruber would hesitate to both report on that and stop taking their ads -- doing so would have little to no effect on his bottom line.
(The observation made by another commenter about how many bloggers and, of course, podcasters pump Squarespace without actually using Squarespace is wryly amusing, though.)