User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20161017030209 Steps to reproduce: I use a lot of bookmarklets (https://github.com/janmoesen/bookmarklets) that take parameters (e.g. "enw mozilla firefox" to load the English Wikipedia). The parameter string %s seems to encode spaces as "+" instead of "%20" now (latest nightlies). mozregression pointed to this pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=7452437b3ab571b1d60aed4e973d82a1471f72b2&tochange=a71215ad8ab820b2310f5dd38fcfded38d18768b Bug 1306639 was the only one that seemed to mention the location bar and bookmarks. Simplest test case: create a bookmarklet with location "data:text/plain,%s" and keyword "test", then type "test foo bar" in the location bar and press Enter. Actual results: Result: "data:text/plain,foo+bar" Expected results: Expected: "data:text/plain,foo%20bar" (Shown in the location bar as "data:text/plain,foo bar")