Currently both the Browser Toolbox and Browser Console are using HTML documents. Using XHTML documents could improve performance for some DOM operations, and chrome page loads should be able to use the prototype cache (Bug 1527977)

FWIW, DAMP does not report any regression or improvement: https://treeherder.mozilla.org/perfherder/compare?originalProject=try&newProject=try&newRevision=697b12000e2bab4f328b6afab150e0d9d5b6094a&framework=12&originalRevision=c6717b080bb5b2fb95df6363487ec14bca85fc8f&page=1

One afterthought, the Toolbox document loads each individual panel in an iframe. So Inspector is in an iframe, console in another iframe etc... A few use xhtml (inspector, storage...), but most of the recent panels use HTML.

We probably have to move them all to xhtml if we want to see an actual impact?