On Friday, the necko team finally landed a fix that makes cache less likely to freeze the UI thread during reads: bug 722034. Cache writes, other less common cache use-cases remain problematic (tracked by bug 717761). Poor cache/main-thread interactions are one of the main causes of UI lag tracked by the Snappy project, so this is very exciting. Barring the need to backout, this fix will appear in Firefox 15.

Help Wanted: The necko team is looking for some help to determine the optimal disk cache size, please see Nick’s post. We need users to install an extension and submit detailed stats on our cache lifecycle.

There are various Firefox frontend fixes in progress: improving session restore (working towards 669603, 669034), FUEL (bug 750454),  search service (bug 722332) and the new theme (bug 732583). I will blog about these in more detail as they land.

Bill landed turned on incremental GC again. Hopefully it will stay on in Firefox 15.

Andrew is making progress on reducing CC pauses while closing tabs: bug 754495.

Brian has instrumented our event loop to measure the extent of Firefox lag when responding to user events, bug 759449. This is different than measuring general event-loop lag in that it focuses on lag that the user would actually notice. Look for  the EVENTLOOP_UI_LAG_EXP_MS histogram in our telemetry dashboard (yes, we are the only browser vendor to make this sort of data public). This should help us track progress as we tweak heuristics to delay background processing during user interaction (eg bug 712478).

Brian also landed a way to bypass the windows prefetch service via our privileged silent update service, see bug 692255. In my testing prefetch is likely to prefetch too many files, slowing down startup for complex apps like Firefox. Hopefully we can do better with our own prefetch.