IT WORKED!.
The beast actually compiled(!)

I have now gotten things to run using the bits built from this effort (including the xulrunner binary it built) and the latest version from the http://conkeror.org/ git. It is slow as molasses for some reason I have yet to identify (top showed it taking up to 90% or more CPU at times) even though it should be an optimized build. And there are definitely bugs. But at least I know that Firefox 24.8.1 DOES run on the Nokia N900.

Now that I know the thing not only compiles but works (to the point I can browse web pages using TLS 1.2 at least) the next step is to figure out why the debian packaging isn't working right and get some .deb files. After that, I can try and compile various bits that use microb-engine and see what happens.

Ported a few patches over from the microb-engine patch set:
002_drawingMissingGlyphs.diff (mozilla bug 463927)
010_MICROB_paths_and_names.diff (this one is what's causing the packaging to fail, now that I ported it the packaging should find what its looking for where it needs it I hope)
050_MICROB_autoset_grehome_display.diff (makes it easier to use xulrunner)
130_456015.certview.diff (mozilla bug 456015)
92422.workaround.diff (mozilla bug 469667)
BMO530075_connectivity_ui_fix.diff (mozilla bug 530075)
BUG153489_increase_max_script_runtime.diff (increase max script runtime)
bug588724.diff (mozilla bug 588724)
connectivity.diff (mozilla bug 532078)
HiddingLibxulSymbols.diff (hides symbols that aren't needed to be exported)
JSBreakExperimental.diff (mozilla bug 449109)
nb162660.diff (changes a function that gets the physical memory size so it only gets the size once and returns the stored size every time)

No clue if any of this stuff actually compiles, I haven't tried it yet but I will do so soon. The next steps are to get this to compile and produce deb files then to look into ABI/API changes, see which users of microb-engine still compile (and what breaks there) and look into getting the gtkmozembed stuff working again (I may need to get that working before I get working deb files)

Oh and for anyone following this thread, DO NOT replace any system binaries with any binaries you may get from building any code specified in this thread. (I learned that the hard way when I had to use RescueOS to put the old contents of /usr/lib/microb-engine back when the stuff built from this new code made the phone go into a boot loop due to browserd being unable to load).

Oh and identifying all the users of the microb-engine bits is also high on the todo list so I know which closed source bits might cause ABI/API issues (the biggest problems will likely be nokia-maps, the various addons/plugins and possibly the closed-source browser UI)

After some analysis I have identified the following closed-source packages as those that would be affected by any ABI changes (i.e. they link directly to microb-engine libraries or talk to microb-engine in some other way)
nokia-maps-core (nokia maps core plugins and libs)
camel-as-provider-0 (provides Microsoft ActiveSync email support for modest I believe)
libssoautologin (single sign on stuff, not sure what for, likely something Nokia/Ovi)
adobe-flashplayer (Flash player plugin)
tablet-browser-default-plugin (default plugin for microb, not sure what this does exactly)
tablet-browser-mediaplayer-plugin (media player plugin for microb)

Getting closer to working packages.
Had to fix a few of the back-ports and remove the back-port for HiddingLibxulSymbols.diff since I couldn't make that patch work (gave me linker errors on libxul)
Just need to figure out what the packaging is looking for but can't find and why it can't find it.

xulrunner-dev ?

I managed to get elfhack working so that's a little progress.

Although I am very close to saying "screw it" and looking for a fork of webkit or similar that works on our ancient libraries but supports the features needed (TLS 1.2 in particular but also more modern HTML/CSS/JS so it can render web pages that microb can't do).

I dont know of anything suitable though...

And what happened then?