Above comments offer no help because they don't know well about Archlinux (yes Manjaro is based on Archlinux).
The cause of this issue is Archlinux's rolling update, while Firefox is still using the old version of
icu, this package has been updated in Archlinux's upstream, so Firefox failed to find
libicui18n.so.58, which is provided by version 58 of
So in order to fix it, just install the old version from AUR via
yaourt or any tool you like. In your case,
icu58 is needed.
And next time you come across the same error, first you should find out which package does the library belongs to, then try to install the required version of that package. In this case,
pacman -Qo /usr/lib/libicui18n.so should tell you
icu own that library, the
.so tells you the required version is 58, then you can go straight to AUR to find that package.
PS: the version number is likely to change over time. I just tested and it tells
*.so.59 not found.