How do free speech protections of permanent positions in the UK/Europe/Canada comparable with those of tenure in the US?
Tenure does not protect free speech, but academic freedoms
Professors can be fired for free speech issues separately.  Tenure in the U.S. system means you've completed a tenure package and been awarded tenure by a panel of tenured professors.
Tenure isn't a free pass to do whatever you want.  It just means as long as you continue doing your job (teaching + research), you won't be fired.  Not getting tenure usually means leaving the university.
There are A LOT of advantages to Tenure (see this question).  It generally means your job is secure, and you have much more freedom to research things others might see as silly or moonshot ideas that may not pan-out for a while.
Free speech isn't protected by tenure.  The university may be more tolerant of a tenured professor stating an unpopular view, but that has nothing to do with academic freedom.  A quick google search will show you if a tenured professor causes enough problems they will be fired.
relevant XKCD

EDIT in response to comments
It's uncommon to fire a tenured professor, but it does happen.  Usually after repeated warnings or obvious abuse of power.
Selman Akbulut was fired from MSU for refusing to teach an undergrad class.
Rick Mehta was fired for controversial statements.
Denis Rancourt was fired for teaching his political views instead of physics.
All were found in a few minutes using Google.