I wouldn't be surprised if retro themes that fit exist for Firefox, though whether
that'll effect window decorations themselves is not clear to me.
Using SeaMonkey is another option, though I probably wouldn't trust it for security
sensitive website logins (such as banks) and would only use it for less sensitive sites
(just out of defensive caution though --- I don't actually know if SeaMonkey has any
real security problems). Konquerer is already similar in that regard though, and already
fits TDE's aesthetics quite well, so I suppose those options (SeaMonkey or Konquerer)
have already occurred to you or are irrelevant to your goal.
Still though, just figured I'd mention the possibility in case you were unaware of
SeaMonkey. Both Firefox and SeaMonkey are developed/maintained by Mozilla, so there is
some degree of commonality.
Likewise, the other person's suggestion of Pale Moon may also be worth looking into.
Oh, but do check if you can change the GTK theme the Firefox may be using! If you have
programs that use multiple different frameworks or libraries or desktop environments then
they can end up with separate control panel programs for altering their respective
appearances. That's one of the trade-offs of allowing multiple different desktop
environments and window managers like Linux does. You may have to manage multiple windows
appearance control pages, each for different subsets of what programs you use.
Anyway, hope that helps at least some. Have a great day/night!