Newest ESR Version of Firefox is detected as outdated
I have the most recent ESR Version installed (atm 45.2.0) When using Google-Mail it tells me I'm using a not supported Browser version. Even the Firefox own site (https://www.mozilla.org/en-US/firefox/new/) tells me my Firefox version is outdated ( Looks like you’re using an older version of Firefox). Is there a reason for this behavior or is this a bug?
Opaite Mbohovái (3)
That is because ESR is not the latest version of Firefox (the latest is Firefox 47 at this time). It is released for organizations that have a dedicated IT support staff to handle deployments and issues like this, but for home use you should use Firefox release, www.getfirefox.com.
As a footnote, Firefox reports your "major" version number to sites, so Gmail sees your Firefox as Firefox 45.0 instead of Firefox 45.2.0. Assuming Google even cares about the difference... it might not if there is some new feature that affects how the site works (ESR got selected security fixes from Firefox 46 and 47, but not new features).
Firefox useragents since 16.0.2 has not shown the minor versions/chemspill updates since so if there was a 45.2.1 version the useragent still shows as 45.0. Same if there is a 47.0.1 for release it would still show as 47.0