I have verified my setup on another laptop; in the hope it might be helpful to someone:
The following packages need to be installed either via 'yum install' or ANPM's Quick install:
netlabs-exp
nasm gcc gcc-wlink gcc-wrc kbuild-make m4 grep gawk perl python findutils tar pkgconfig perl-PathTools pthread-devel
exceptq exceptq-devel autoconf213 autoconf nspr-devel nss-devel libicu-devel pixman-devel cairo-devel pango-devel
fontconfig-devel freetype-devel libkai-devel libvpx-devel libjpeg-turbo-devel libpng-devel zlib-devel bzip2-devel
hunspell-devel libcx-devel expat-devel python-test ffmpeg-devel libtool libtool-ltdl-devel gettext-devel
git
mercurial
Install the OS/2 Developer's Toolkit, this may be done with yum, too.
In the config.sys set VIRTUALADDRESSLIMIT=3072.
Reboot, so changes applied to your config.sys become active.
Install lxlite from
http://home.earthlink.net/%7Esteve53/betas/lxlt135-shl-20121201.zip into your PATH.
Follow Dave's instructions to download the sources:
https://bitbucket.org/dryeo/dry-comm-esr31/src/ad7137973e042eb45ca5fb35764ebafb2db40f4c/BUILD.OS2?at=default&fileviewer=file-view-default. If you get an 'File or directory not found' error it might be possible that you have to set HOSTNAME in your environment settings.
Download all attached files, edit them so the paths match your environment. Move config.site to \usr\local\share.
Open a command shell in \cc-31\mozilla, execute sh.exe in this command shell. In sh execute the following:
autoconf213
(cd js/src && autoconf213)
(cd js/src/ctypes/libffi && autoreconf -fi)
(cd nsprpub && autoconf)
In /cc-31 open a command shell (use cmd.exe, not 4os2, and pay attention that the environment setting COMSPEC is set to C:\os2\ cmd.exe). Run mozenv.cmd and build_suite.cmd. After the build process completes run smxqs.cmd, cd into obj-sm and execute 'make package'.
The zip package is in \cc-31\obj-sm\dist.
The build process took about 5 and a half hour on a laptop with an Intel CoreDuo 1,83 GHz and 2 GiB RAM; 8 and a half hour with a Pentium M 2,13 GHz and 1280 MiB RAM.
This instruction might be wrong, incomplete or completely useless. Especially it is likely that some of the RPM packages are not necessary.
Thanks to everyone, who helped me finding out the steps.