I'm going to answer your second statement first. The subject pulldown works fine with FPR8, I just tested it and successfully posted an issue as an unverified new user. This response is being typed on a quad G5 in Tiger. As to why your system isn't showing it, the usual culprits likely apply such as malfunctioning addons, or a bad profile. Usually I advise creating a new profile and seeing if the problem manifests there too. See "How to reset your profile" at lower right.
You can also cut out the "lame" crap. There's exactly one full-time developer whom you don't pay, who actually has a life and a day job. I appreciate bug reports, and there are certainly issues with the browser, but I don't appreciate snark. If that level of support isn't adequate for you, then use something else.
TenFourFox doesn't support webextensions, only "classic" addons compatible with Firefox 45. If Startpage doesn't provide such an addon, "someone" will have to write one. I don't use this service and the core browser is a higher priority, so it isn't likely to be me. If you decide to do it, I'll be happy to make it available.
I figured out how to do it, and it actually turns out to be rather easy, it's just not apparent. You have to go to the search engine's page first, then click the Plus sign in the search field, and then it will show up in your list of available choices and you can select it as your default search engine.
I'm sorry you were offended by the fact that I found it lame that I could not create a topic about Tenfourfox while using Tenfourfox itself. You have to appreciate the irony there, and feel free to substitute that word instead.
I don't have any addons, so I don't think that's the issue. I suppose it could be a "bad" profile - literally anything is possible.