0. Albeit there' s no need to move the NSS/NSPR DLLs to the tree of the Mozilla product w.r.t. InsTurbo, as long as it's installed in a CONFIG.SYS LIBPATH directory. If your install doesn't need it, then you can also copy any file to a missing DLL ("DIR > missing.DLL") if ABOVE512.EXE or HIGHMEM.EXE will just report an error ("Not a valid LX DLL") and continue. If your FF/SM/TB doesn't require a DLL, then you don't have to install it to please nothing but InsTurbo.
1. Perhaps a missing DLL also means a version of FF/SM/TB which is too old to be a default browser, because BEGINLIBPATH (not supported by WPUrl) is an implied recommended requirement then? If so, then the error text could be changed to e.g. "DLL(s) not found, or FF/SM/TB is too old (because apparently it doesn't require a newer DLL)".
2. Can a missing DLL become a warning instead of an error? I guess this depends on what you want to support. As such I have no problem with the fact that InsTurbo checks the setup, with a DLL-related error instead of a DLL-related warning. That was and is the current approach.
3. I've attached a first sample file, for any MOZILLA.EXE (and, optionally, MOZTURBO.EXE) only. My copy of Mozilla has a chrome\icons\default directory too, but in the case of FF the files can be moved to the directory of FF too. The file INSTURBO.ERR is a reserved file name, which may be deleted by INSTURBO.EXE. VROBJ.DLL is an usual, obvious requirement.
Unzip it in an empty directory, and execute INSTURBO.EXE
If there's any reported missing *.EXE file, then execute e.g. "DIR > MOZILLA.EXE" or "DIR > MOZTURBO.EXE" at the prompt and try again. Or copy a real *.EXE file. The *.EXE files just have to exist. ABOVE512.EXE or HIGHMEM.EXE aren't involved (no MOZILLA.EXE DLLs compiled and supported by Dave Yeo).
It may create the 3 WPS objects (2 at desktop, 1 in startup folder), and it may change OS2.INI even when the current WPUrl defaults are (a different) MOZILLA.EXE and/or working directory. If you don't want OS2.INI changes, then don't answer "Yes" during the confirmation of that setting.
When done, delete the 3 WPS objects, all files in the former empty directory (RM * -R), and restore the previous OS2.INI WPUrl default settings. By restoring OS2.INI, or by executing MOZTURBO.EXE in the directory of the old MOZILLA.EXE default browser.
There's no need to perform a full test, but of course any possible issue can be reported.