Firefox freezes for several seconds at first start

In case your Firefox profile resides on a network share it might freeze for several seconds at first start or randomly while using. To prevent this add the following to your .xsessionrc

[[ $(df -P -T "${HOME}" |tail -n +2 |awk '{print $2}') =~ ^(nfs|smb) ]] && export NSS_SDB_USE_CACHE=yes

If you intend to start firefox from the command line add the above line also to your .profile

The proposed addition to the release notes of Firefox 60 ESR summarizes the problem as follows

In versions Firefox 58 and later, and in Firefox 60 ESR and later, Firefox uses a new format for storing private keys, certificates and certificate trust.
Users, who store their home directory or application data directory on a network filesystem, are advised to set the environment variable NSS_SDB_USE_CACHE=yes prior to starting Firefox.
Without setting this environment variable, users of network filesystems may experience a major slowdown when starting and using Firefox.