“If you’re on an older version of Android, we recommend you install Firefox Mobile, which supports Android 5.0 and above as of the time of writing.
Why does installing Firefox help? For an Android phone’s built-in browser, the list of trusted root certificates comes from the operating system – which is out of date on these older phones. However, Firefox is currently unique among browsers – it ships with its own list of trusted root certificates. So anyone who installs the latest Firefox version gets the benefit of an up-to-date list of trusted certificate authorities, even if their operating system is out of date.”