That's not at all fair. I, along with many others, followed his advice and I was thereby able to access my email, which I hadn't been able to previously.
Using Firefox is by no means perfect, as I found that trying to open other web pages at the same time as Outlook often caused them to crash. I therefore ended up running Chrome for most browsing, and Firefox exclusively for Outlook. But having two browsers working at the same time slowed my ancient PC down to even less than it's usual snail's pace.
Also, the current version of Firefox needs Windows 10 as a minimum, but you can use version 115 with their Extended Support Release, though the ESR will expire in September - https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support
Sad to say, Microsoft and Google have successfully conspired to make sure that users of any version of Windows before W10 will really not be able to use Outlook 365 for much longer, and will ultimately be forced to upgrade.