Hey, thanks, that fixed it! By this:
cd /tmp
git clone https://github.com/MoonchildProductions/UXP
cd UXP/application/
git clone https://git.hyperbola.info:50100/software/iceweasel-uxp.git
wget https://paste.debian.net/plain/1081986 -O iceweasel-uxp.locale.patch
cd iceweasel-uxp
git apply ../iceweasel-uxp.locale.patch
cd ../..
wget http://oscomp.hu/depot/iceweasel-uxp.mozconfig -O .mozconfig
./mach build
./mach package
cp ./obj-x86_64-pc-linux-gnu/dist/iceweasel-uxp-*.tar.bz2 /opt/
cd ..
rm -rf ./UXP
cd /opt
tar -xjvf iceweasel-uxp-*.tar.bz2
rm iceweasel-uxp-*.tar.bz2
i now have a portable Iceweasel-UXP in /opt. Thanks again for the help.
BTW, i also have Iceweasel 38.8 installed and it seems, that Iceweasel-UXP (52.9) shares config paths with it, because it took all the old IW-s' extensions and settings. Not my decision and not a problem for me, but i think it should not do that, since IW 38 has old extensions and IW-UXP 52 has new ones and if i would update them, then IW 38 cannot use them. While it is true, that now i could actually retire IW 38, still it can be useful for testing or other reasons to have another separate usable and trustable browser.
I've read this topic, but it is not clear to me, if this patch changes this, or just the app name. Also, if it's the first one; will it make into to the tree, or it will remain as a separate patch?