I spent last week trying to reproduce slow startup on Windows. Some users were reporting >30 second startup, supernova_00 has been feeding me xperf traces on IRC reproducing slow startups.
Startup Bugs
Turns out that if a website uses non-standard font names this can trigger Firefox to start parsing every single font on the system, freezing the browser in the meantime. Turns out facebook does this 
. This is now a blocker bug 600713. This bug has the unfortunate effect of overshadowing any startup improvements in Firefox 4.
We have some code to keep our databases in good shape by VACUUMing them. This is getting revamped in Firefox in bug 541373. In the meantime, for many current users performance suffers due to missing vacuums. If you are suffering from slow Firefox startup, and/or slow Awesomebar try this manual vacuum. This helps in older Firefox releases, but in Firefox 4 this has the effect of supercharging the SQLite database performance by switching to 32K pages.
Scareware
Another fun discovery was the effect of anti-virus software(AVG in this case). Like an annoying pet, AVG has to have a sniff and fondle every file that Firefox opens on startup. Apparently this is a feature called on-demand scanning, yuck.
But the fun doesn’t stop there, Windows has a wonderful prefetch mechanism that speeds up app startup. Unfortunately for supernova_00, \Windows\Prefetch just wouldn’t get populated with Firefox info, meaning that Windows wasn’t optimizing Firefox startup. Once I installed AVG, I ran into the same problem. Uninstalling AVG didn’t help. For whatever reason deleting every file in \Windows\Prefetch fixes that problem. For both of us prefetch got repopulated after being cleaned.
XPerf
Microsoft XPerf makes trivial to optimize cold startup. None of the other OSes have precanned analyses showing how much each individual file access is contributing to slow startup.
If you have a startup problem, I’m much more likely to be able to reproduce it if the report comes with an xperf trace. To get xperf run the Microsoft Platform SDK installer, select “Windows Performance Toolkit”.
To record an IO trace:
- Reboot
- run cmd.exe as Administrator
- xperf -on latency+FILE_IO+FILE_IO_INIT+DISK_IO
- run Firefox, reproduce the bug
- xperf -d report.etl
- Run xperf report.etl to view the report.
Click on “IO Counts” or “Hard faults” graph, select “Summary Table”. “IO Time (ms)” is the interesting column there. To get an idea of the sequence of IO operations, export the summary table to .csv and load it in a spreadsheet/grep/whatever. Every Firefox developer should give xperf a try, addon authors are encouraged too.