The trouble is that unfortunately there really isn't really any straightforward, objective, useful numeric measure of how "secure" one browser is compared to another. Or, really, how secure one piece of software is compared to another more generally.
Want to compare gross vulnerabilities reported & fixed during some period of time? Well, as the commenters pointed out there are several reasons you actually might not want to use total vulnerabilities as a representative measure of the practical security of a browser. Which means that comparing that number for one browser to that number for another is even more problematic. (For example, comparing Chrome's number of reported/fixed vulnerabilities to any other browser's.)
Well then, what about the number of vulnerabilities successfully exploited in the wild over some length of time. Well, that's perhaps a less-bad measure than total vulnerabilities in some ways, but it still has substantial problems that make it quite flawed as a basis for comparison. Browser A may have more exploited vulnerabilities than Browser B, but what if that's because Browser A garnered more attention from the security researchers, criminals, and/or nation-states who created exploits for vulnerabilities than Browser B did? Which, in turn, could be due to any number of reasons not related a browser's "inherent" security or lack thereof. (A real world example: it's likely true that one reason IE has historically drawn more exploitation attention than Firefox is because it is more heavily used within corporate and governmental sectors, which , of course, is where the greatest number of systems that are most attractive for attackers generally are.)
Or you could look at, say, the number of in-the-wild zero-day exploits (ie. vulnerabilities that became known & exploited before a vendor could issue a patch) that bedeviled a browser over a certain timeframe. But that measure has issues similar to those just discussed. And do you count exploits that allowed breach of a browser's site-rendering element but don't allow escape from that browser's sandbox?
Or you could look at...
But I think you're getting my point. There really isn't any one number that really captures a browser's susceptibility to compromise well or allows good comparisons on that basis. Which is essentially what an effort to compare announced vulnerabilities is trying to get at.
So, does that mean we simply can't compare security among browsers at all? No, I certainly wouldn't go that far. There are points of comparison that, if more abstract than numerical measures, are valuable. For instance, IMHO the architectural & defense-in-depth attributes of some browsers give them some significant advantages in security over others. (Though we mustn't forget that how a browser is configured and used is perhaps an even more important factor than the software's security architecture.). But straight numbers vs. numbers comparisons can be quite misleading.