Go to the download page and check download link - it contains the version number. You can also check the Release notes on the above page.

Note that there have been issues reported with Firefox 55.0 that need investigation. There is a Firefox 55.0.1 update available for Firefox 55.0 users that addresses a few issues. For Firefox 54 users updating to 55 has been disabled until all know issues have been addressed.

If you aren't using a Windows user account with a quote (') in the file path to the Firefox profile folder then you can consider to update to Firefox 55.0.1