In the FAQ section, the answer to “Can I use Firefox Sync with LibreWolf? Is it safe to do so?” is that “There aren’t any major privacy or security downsides in doing so, as Firefox Sync encrypts your data locally before transmitting it to the server”.
So why does LibreWolf disable it by default?
it probably takes some getting used to, yeah. however (I maintain the settings) I try to trim the number of about:config changes needed as much as possible, so that it’s not annoying for users.
It was the first I enabled.