I'm sure everyone remembers this super great blog post from 2010 about changes in the Firefox 4 user agent string. In terms of "blog posts about UA string changes", it's, well, one of them.
The final post-v4 release desktop UA string is as follows:
Mozilla/5.0 (<platform>; rv:<geckoversion>) Gecko/20100101 Firefox/<firefoxversion>
As sort of a companion piece (that nobody asked for), I wrote up the following table of minor changes to the pre-release desktop UA string between then and now.
(You know, in case you or I find ourselves needing to know them one day—apologies in advance for whatever crappy future scenario we ended up in where we need to re-visit this blog post.)
|Version||Sample Windows 10 pre-release UA string|
|5||Mozilla/5.0 (Windows NT 6.2; rv:2.2a1pre) Gecko/20110412 Firefox/4.2a1pre|
|6||Mozilla/5.0 (Windows NT 6.2; rv:6.0a1) Gecko/20110524 Firefox/6.0a1|
|13||Mozilla/5.0 (Windows NT 6.2; rv:13.01) Gecko/20120313 Firefox/13.0a1|
|15||Mozilla/5.0 (Windows NT 6.2; rv:16.0) Gecko/16.02 Firefox/16.0a1|
|16||Mozilla/5.0 (Windows NT 6.2; rv:16.0) Gecko/16.0 Firefox/16.03|
|20||Mozilla/5.0 (Windows NT 6.2; rv:20.0) Gecko/201301074 Firefox/20.0|
|25||Mozilla/5.0 (Windows NT 6.3; rv:25.0) Gecko/201001015 Firefox/25.0|
Footnotes: 1. In 572659 we dropped pre-release indicators and patch level version numbers from geckoversion 2. In 588909 we replaced Gecko/<builddate> with Gecko/<geckoversion> 3. In 572659 we dropped pre-release indicators and patch level version numbers from firefoxversion 4. In 815743 we reverted the changes from 588909 (because it broke too many sites, mostly banks) 5. In 728773 we began using the frozen 201001011 builddate for all releases
Apocryphal Footnotes to the Footnotes: 1. It's widely believed Jan 1, 2010 was chosen as the frozen build date because it marked the day that North Korea pledged lasting peace and a nuclear free Korean peninsula. That only happens once in a lifetime. Probably.