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Browser version testing

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I am building a new website and looking at traffic to my existing site there is a range of FF versions being used. The traffic is mostly from versions 35 and up.

With these multiple versions and new ones coming all the time, what versions of FF do I need to test my new site on? Is there a point at which backwards comparability can be assumed?

I am building a new website and looking at traffic to my existing site there is a range of FF versions being used. The traffic is mostly from versions 35 and up. With these multiple versions and new ones coming all the time, what versions of FF do I need to test my new site on? Is there a point at which backwards comparability can be assumed?

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You should not be seeing Fx35, if you do it is no longer supported by Mozilla, neither are Fx36, 37 ,39 ,40 ,41, 42. Note the gap Fx38.X esr is supported Fx43 is not supported, but you are likely to see it.

Every ordinary user should be on the Current Release. That updates every six weeks. However obviously not everyone updates immediately, even though by default Firefox is set to automatically update.

Large institutions use a Long Term (ESR) version, that has a slower major update cycle and longer overlap as explained at:

So you should test the latest two point issues of ESR (Currently Fx38.X) and whatever the current Release is (Currently Fx44.0.x). Noting that bug fixes etc may be released hence we are now on Fx44.0.2

To keep ahead of the game you should consider also testing the Beta and maybe the DE(was Aurora) versions. You will have no need of using the Nightly version unless you are investigating current bug fixes or features not yet available elsewhere.

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This is very helpful information, hower Fx38, 43 and 44 accounts for only 75% of Fx traffic to my site in the last month. The other 25% is distributed across many versions so I guess there is a pretty decent number of people who are not auto updating.

Also, my release/testing cycle for my site will not follow the same cycle as Fx (or Chrome, Safari and IE).

What I'm really looking to understand is what level of change/backwards compatibility can be expected from version to version? Having to test such a frequent release schedule for all the browser types and versions is just not sustainable for me

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Which version(s) do you test in now?

You can use this site to assess whether a particular version has any headline changes affecting sites:

Firefox 44 Site Compatibility - see right column for each previous version

If you are using any modern tags, style rules or APIs, there often is a browser compatibility table at the bottom of the corresponding article on MDN. For example: https://developer.mozilla.org/docs/Web/API/notification#Browser_compatibility