I just tried the 64 bit slackbuild script from
http://slackbuilds.org/repository/15.0/network/icecat/
and it runs on a Slackware 15.0 64bit fresh installation but I'm getting some font foolishness (e.g. on this forum, thread titles not displayed but tooltips still working). No error messages about missing libraries or anything. I have a small number of slackbuilds on this laptop so it isn't 'clean'.
The SlackBuild is repackaging a binary not compiling from source. I made sure to download the icecat-60.7.0.en-US.gnulinux-x86_64.tar.bz2 binary.
The library you mention is as you say in lib64...
Code:
bash-5.1$ locate libgmp.so
/usr/lib64/libgmp.so
/usr/lib64/libgmp.so.10
/usr/lib64/libgmp.so.10.4.1
As Icecat 60.7 is basically Firefox 60.7 with some proprietary stuff taken out, yes many sites won't work well with it. I personally use Seamonkey and occasionally fall back to firefox 91 as supplied with the 15.0 install.