To be as candid as possible, there is no guarantee TenFourFox will continue; there never has been. Part of my reasoning for not accepting money or even donations is so that no one has that expectation. So don't use this as the key point in making your purchase decision -- even though Power Macs are still getting cheaper to acquire used, the supply is getting near peak availability and they are getting more interesting to collectors as they get older, and my suspicion is that the price will bottom out in the next year or two. I think that would be more useful to figure into whether you buy now, later or not at all.
That said, the primary purpose of the browser is so that I personally have one to use myself. Since I use a Quad G5 as my primary computer as well, I support it because i have self-interest in it. I don't anticipate getting a new system in the next year or two, so you can reasonably expect the software to be supported at least that long, and even on that unhappy day when the G5 is no longer meeting my needs I will probably do a security-patches-only period where the browser is not being given new features but will still be safe to use to allow people a transition period. But, again, that's a plan; that's not a promise.
The source code is of course available from Github with build instructions. If you find you have a strong dependency on the browser, it might not be a bad time to learn how to build it yourself. That way you'll have recourse instead of having to depend wholly on me, and you could consider making your own updates. Just a thought.
Meanwhile there are no imminent plans to discontinue browser support.