Ok so I have identified that microb-engine is using a Gecko revision taken from somewhere in the mozilla-1.9.2/Firefox 3.6.x tree and is using NSS_3_12_6_RTM.
The TLS 1.2 support didn't land until NSS 3.15.1 and the first version of Firefox to use that is Firefox 24.

Based on what I have observed, we have several options here as a way forward:
1.Start from the mozilla-central commit for NSS 3.12.6 RTM, move forward in time and look for any commits that update NSS (or that are necessary in order to update NSS to newer versions) and try and back-port those changes to microb-engine

2.Take the Gecko base code from Firefox 24.x and figure out if we can make it work as a replacement for microb-engine ([porting various local microb patches to that code, identifying if anything has changed API or ABI wise in the header files that get used by the stuff outside of microb-engine, figure out if any maemo-specific bits have been removed from the codebase etc) then if we can make it work (either as-is or with some porting work on the things that talk to microb-engine) use that to replace microb-engine.

or 3.Try to get Fennec 24.x working on Maemo5 and modify it so it can slot in as the "system" browser but with microb-engine remaining around for maps and rtcom-messaging-api and such (again identifying if there are any bits removed that matter, any maemo-specific changes we need to make/port across/whatever). I dont know when Fennec dropped all the bits needed for Maemo/Linux and became "Firefox for Android" but I have seen a Fremantle port of Fennec 17 so that might be a place to start.

The goal of this work is to allow people who are using Maemo Fremantle (and who are interested in the work) to have a browser that can at least connect to web sites using the latest security protocols and (depending on how things are done) maybe also get some browser improvements in there as well so it can do better on rendering web pages.

There IS an option 4: Find all the patches needed for TLS 1.2 support in NSS and mozilla and back-port those to microb-engine somehow.

Oh and for those who say "forget about microb and stuff and use an existing modern replacement", what modern replacement do you suggest people on Maemo Fremantle use?

The newest Fennec build currently available for Maemo is Fennec 17 and that's not new enough to support TLS 1.2 and the newest security stuff so there is no existing Gecko browser that we can use.

All the new-enough webkit based browsers out there would need just as much porting work to run on Maemo as a newer Gecko does so that's not an option. And Leste isn't usable as a daily driver yet (and doesn't have a browser yet anyway) so switching to a new OS is also not an option. (and obviously "get a new phone" isn't the answer either since people using the N900 in 2018 are almost certainly using it because they like it and prefer it to iOS and Android :)