chromium-browser package in Ubuntu's LTS repos is rather old for the older 12.04LTS & 14.04LTS. See this search of Ubuntu's packages for chromium-browser to see that chromium-browser is at:
- version 37.x in 12.04LTS (Apparently from June 2014)
- version 53.x in 14.04LTS (Apparently from June 2016)
- version 55.x in 16.04LTS through 16.10
Whereas Firefox is at version 51.0.1x in every currently supported Ubuntu release from Precise (12.04LTS) to Yakkety (16.10) (actually, Zesty 17.04 has version 50, but it's still March so it's not really released yet).
The official Ubuntu repositories are generally the best place to get the latest supported versions of everything, and using apt/synaptic/software center is generally the best way to update software (supported by this similar Q How to install the latest stable version of Chromium?).
Especially considering using an old web browser is generally a bad idea, and Google says that Gmail (and probably other Google apps too?) will no longer support Chrome (essentially Chromium) versions 53 & older past 2017, why have the older, still-supported Ubuntu LTS's apparently given up on Chromium?
PS. I'm not asking about why "packages in the official Ubuntu repositories [are] older than the latest (upstream) versions from Debian Sid, PPAs, the authors, etc.?" (related Q: Why don't the Ubuntu repositories have the latest versions of software?), but rather why the latest versions from Ubuntu itself aren't updated, while Firefox is.