Firefox 138 was released yesterday, and aside from enhanced profile management and tab group support, it doesn’t have any particularly exciting features and is a pretty basic version. That's not the case with Firefox 139, which is currently in beta and does bring some noteworthy features, such as faster HTTP/3 upload performance.
Firefox 139 beta has faster upload performance over HTTP/3, even to the point of "significant improvements." Especially for HTTP/3 connections involving QUIC 0-RTT resumable connections and high-bandwidth and high-latency connections, HTTP/3 upload speeds should be faster on Firefox 139 than in previous versions.
Some other improvements in Firefox 139 Beta include support for full-page translations on Firefox extension pages; PNG images with transparency are now preserved when pasted into Firefox; and support for the "hidden=until-found" attribute, which allows content to be found through the Find in Page feature but not displayed by default.
You can get more details about the Firefox 139 Beta version via Mozilla.org.
Download address:
https://ftp.mozilla.org/pub/firefox/releases/139.0b1/