Using 64bit nightly on windows. My profile has around 1500 tabs, they are lazy-loaded and spread over ~10 tab groups. Profiler output of a slow startup, with all addons except the gecko profiler disabled: http://people.mozilla.org/~bgirard/cleopatra/#report=b080c0b5196f85d57541ca9ceab5cfbb61ef3980 BenWa already identified bug 989816 as root cause for 30 out of those 73 seconds. The remaining time seems all over the place, but largely related to creating/restoring tabs.

I experienced same slowness under Debian Firefox/Iceweasel v42.0-1 AMD64. With near ~1000 tabs it opened for 1 minute, but with ~1500 it takes 2-5 minutes. With 1600 it work very hard. I report another issue on my experience when work with many tabs: https://bugzilla.mozilla.org/show_bug.cgi?id=1233464 I profile with geckoprofiler-signed.xpi and upload reports to https://cleopatra.io/ but I don't know how to profile during startup. Can any provide instructions?

> but I don't know how to profile during startup. press the "firefox restart" button in the profiler, that should restart a browser with profiling enabled

This seems like a duplicate of Bug 1025924

Hi The 8472, can you re-test and see if this still happens or not?

Lazy virtual tabs have solved this issue.

In what version/build I should expect to see bug fix?

55 according to bug 906076