On startup, and to a lesser extent when browsing, TenFourFox (and Firefox) enumerates the fonts you have installed on your Power Mac so that sites requesting them can use locally available fonts and not download them unnecessarily. The reason for periodically rechecking is that people can, and do, move fonts around and it would be bad if TenFourFox had stale font information particularly for commonly requested ones. To speed this up, I actually added a TenFourFox-specific font directory cache so that subsequent enumerations are quicker. However, the heuristic for determining when fonts should be rescanned is imperfect and when in doubt I always err towards a fresh scan. That means a certain amount of work is unavoidable under normal circumstances.
Thus, the number of fonts you have currently installed directly affects TenFourFox's performance, and TenFourFox is definitely not the only application that needs to know what fonts are installed. If you have a large (as in several hundred) number of font files and particularly if you are not using an SSD, you should strongly consider thinning them out or using some sort of font management system. Even simply disabling the fonts in Font Book will help, because under the hood this will move the font to a disabled location, and TenFourFox and other applications will then not have to track it further.
How many is too many? On my quad G5, I have about 800 font files on my Samsung SSD. This takes about 3-4 seconds to initially populate the cache and then less than a second on subsequent enumerations. However, on a uniprocessor system and especially on systems without an SSD, I would strongly advise getting that number down below one hundred. Leave the fonts in /System/Library/Fonts alone, but on my vanilla Tiger Sawtooth G4 server, /Library/Fonts has just 87 files. Use Font Book to enable fonts later if you need them for stuff you're working on, or, if you know those fonts aren't ever being used, consider just deleting them entirely.
Due to a work crunch I will not be doing much work on FPR16 until August. However, I will be at the Vintage Computer Festival West again August 3 and 4 at the Computer History Museum in Mountain View. I've met a few readers of this blog in past years, and hopefully getting to play with various PowerPC (non-Power Mac), SPARC and PA-RISC laptops and portable workstations will tempt the rest of you. Come by, say hi, and play around a bit with the other great exhibits that aren't as cool as mine.