Not necessarily. I am a web dev, testing is fucking HARD. If you dont HAVE to do it, you usually dont. Testing that your website works perfectly on all browsers is a huge pain, in basically all scenarios. I do my development in one browser, and I do not check other browsers, it happens occasionally that I will be contacted and told my code doesn't work in y browser, my first instinct is, well it works in x. Can you use x?

It is entirely possible that google is incentivized to give older versions to not chrome to encourage people to use chrome, but that doesn't sound realistic to me. There are a couple problems with that inherently, using google services gives more value to google than using chrome browser, if you discourage users of other browsers to use your services you arent getting people to switch to your browser, more likely you're losing people who would use your services. Also it's just bad for brand rep, and google lives on brand rep first and foremost.

The realistic scenario is that google controls both its services and chrome fully. They have a capacity to write solid unit tests against their services specifically on chrome, and all of their developers probably develop using chrome as their browser. Google will be able to confidently declare that a release is ready for chrome, not so much on firefox. They dont have control over firefox's source code, they cant create unit tests for it, so it takes actual testing to ensure they arent releasing a buggy product that will damage their rep.

In my opinion this is probably not malicious.