Bug 467890 added a "used" annotation in engineMetaDataService to support tracking the first-use of default search plugins, so that we can trigger an update check. We haven't made use of that functionality (bug 511017), and in the mean time, this is causing us to hit the disk the first time a default search engine is used. and also increasing the odds of needing to write metadata, since it's the only use of the engineMetaDataService that's triggered by a common user action (simply using the search bar).
I think we should get rid of the "used" attribute for the moment. It's easy to re-introduce if we ever do end up investing in the work required to complete bug 511017.
The changeset that added it:
Created attachment 625906 [details] [diff] [review]
Comment on attachment 625906 [details] [diff] [review]
Created attachment 626686 [details] [diff] [review]
Patch for checkin
Pushed to try and waiting for results