I'm not sure it is really - the OS is so old now it could be wide open to viruses and malware and I wouldn't know.
Having said that, I haven't had any problems for years and I use Little Snitch so if some malicious software was trying to send out information I assume Little Snitch would ask my permission first.
But my main question here was - why can't I find any way to browse new extensions or add ons in TenFour Fox? Has it been completely disabled?
Yes, it has been completely disabled. The tab (both in TenFourFox and Firefox) simply redirects to Mozilla's add-on site; there was no special client-side support. Since Mozilla no longer hosts add-ons compatible with TenFourFox (and, for that matter, other Firefox derivatives that do not use WebExtensions), it actually causes more harm than good. Creating a separate add-ons browser for these older add-ons is a much bigger task than I am willing to undertake, and add-on support in TenFourFox has always been "best effort," not guaranteed.
However, if a site offers an add-on, you can still install it by downloading the .xpi and dropping it on the TenFourFox window.