We spend plenty of effort is spent on both stability and performance. "Improved performance by x% in this particular situation that you may or may experience!" just isn't something you can wrap up neatly into a high-level blog post like you can with a new feature, so you may not get an accurate picture of where development effort is going based just on reading the official blog and release notes.
For instance, somebody just landed a pretty tricky bit of code to fix a severe incremental GC performance fault he'd been experiencing with a lot of tabs open (disclaimer: I reviewed the patch): https://bugzilla.mozilla.org/show_bug.cgi?id=1214961
For instance, somebody just landed a pretty tricky bit of code to fix a severe incremental GC performance fault he'd been experiencing with a lot of tabs open (disclaimer: I reviewed the patch): https://bugzilla.mozilla.org/show_bug.cgi?id=1214961