Although he didn’t mention it, you ought to give
@wicknix ’s fork of TenFourFox,
InterwebPPC, a go. And if you want even less of a browsing drag on your system from resource-intensive advertising, installing the add-ons
uBlock Origin Legacy and
uMatrix to InterwebPPC (or even TenFourFox) can help go a long way to make browsing on your PowerBook a less sluggish experience.
Over here, I’m still using my 17-inch and 15-inch PowerBook G4s (also 1.67GHz) daily (one of them, in fact, for testing experimental software). While they’re no speed demons compared alongside a Silicon Mac, they still hold their own remarkably well. Audio editing and imaging software still do quite nicely on them.