this, just to clarify Amin's reply, pedantically - it is important to note that there is no: "the linux package manager" - every distro strain has it's own package manager and packages; which are maintained by that distro and generally usable only with that package manager - in this context, "linux" is but one package among thousands - it is usually moot helpful to refer to your operating system by it's formal name, such as arch, debian, ubuntu, fedora, suse, etc
most of the FSF-approved distros have icecat already; so the simplest solution, is to use one of the FSF-approved distros, instead of the one you are using now - for other distros, it is upon users to request to their distro maintainers, that icecat be packaged - in the case of the mint->ubuntu->debian strain, there is a packaging request open for several years; but it has yet to be accepted into debian - AFAIK, fedora is the only distro which is not FSF-approved, which has an icecat package for their users
