Following up on my previous post on Firefox 38 compatibility, I wanted to highlight an important change that I missed: in-content preferences. The preferences window is no more, and instead the main preferences UI is shown in a new tab. This only affects add-ons that overlay the preferences window, which should be rare.

Additionally, there’s an out-of-cycle release planned for June 2nd, which will go with version number 38.0.5. This is 38 with a few additions, which you can see in the release notes. These additions shouldn’t conflict with extensions compatible with 38, but it could conflict with complete themes. The new release is now available in the beta channel, so we recommend you test your add-ons on that version.

https://blog.mozilla.org/addons/2015/05/13/compatibility-update-firefox-38-and-38-0-5/