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I use my G5 as my main machine at the moment and have been a few months now. I do use TenFourFox at times but it stresses my CPU and makes everything slower. Also I watch a lot of YouTube, which Safari 5 and Flash 10 handles great. TenFourFox with YouTube's HTML 5 player doesn't work at all. I've also had no security issues.
The default configuration of TenFourFox is plain Mozilla. Leave it like that without optimizing it and your complaint about it being slow and spiking your CPU will continue.

Optimizing it by tweaking some settings in about:config will go a long way to resolving your complaint about it. See the link in my signature.

Alternatively, Altemose has made it ridiculously easy for the users by making all the modifications himself in a default prefs.js file and uploading it. If you want to go that route, try his thread here:

https://forums.macrumors.com/threads/easy-install-eyoungrens-tenfourfox-tweaks-download.1892242/

P.S. I also use Safari for Flash. T4Fx, past version 17 has plugins off by default and trying to get old versions of Flash working easily with T4Fx is way more trouble than it's worth.
 
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