There are at least two aspects of your question: is $55000 good for a physics postdoctoral fellowship in Canada; and is $55000 a decent salary to live on in Canada (regardless of profession)? I will answer based on my past experience as a business professor in Montreal (though I will tailor my answer for physics). Right off the bat, though, I will note that your question has nothing to do with the United States, since cost of living issues vary widely not only between adjacent countries but even between cities in the same country. (Actually, I have lived in several different cities in both countries.) I will only answer for Canada.
Is $55000 good for a physics postdoctoral fellowship in Canada? A good point of comparison is the amount of the fellowship awards granted by NSERC (Natural Sciences and Engineering Research Council of Canada), which is the government agency that grants postdoctoral fellowships in physics, among other fields. Here are two key points of comparison:
The most frequent amount that NSERC awards is for their standard Postdoctoral Fellowships Program. This should be considered "good", since that would be the standard point of comparison if a university were to fund its own program without a government grant. This amount is $45000. Although universities might pay someone less than this amount, the NSERC award amount is considered a benchmark.
An excellent amount is the one that NSERC awards for the extremely competitive Banting Postdoctoral Fellowships Program. This program awarded 23 fellowships in the last year (not in physics, but in all science and engineering fields). You would almost never see any university fund a postdoctoral fellow that amount, or anywhere near it, unless the recipient is a Banting Fellow. This amount is $70000.
Based on this, I would consider that, YES, $55000 is very good for a physics postdoctoral fellowship in Canada funded directly by a university.
But, is $55000 a decent salary to live on in Canada (regardless of profession)? This is a cost of living question. My best source for such information is Numbeo, which lets you compare cost of living city by city, which is the most meaningful comparison. I used it a couple years ago when I moved from Montreal to Paris, and I found its estimates to be very accurate and extremely helpful in my financial decisions and negotiations.