I'm curious to know why 99 and not the current version? You can download, unzip, and run firefox-bin from the folder. https://ftp.mozilla.org/pub/firefox/releases/

Thanks for quick response. I need at least 99 for my bank to be happy, but higher/newer is better I'm sure. If I follow your advice, will I lose anything in the process? Please forgive my stupid questions, but I used to rely on my Computer Guru Brother for all things computer, but he's now in the ethers with all things spiritual and I'm here to fend for myself. Many thanks for your help!

I understand your situation and I'm sorry for you loss. You want lose a thing because you are running a different browser. This is not installing so it will not have your data from your current browser. You can also do the same with the current version of Firefox. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

How old is your computer and what are the specs? We get users complaining about resources when they're on old computers.