Devtools slow and occasionally unresponsive on some sites

| Any news on this front […] Using FF 57.0.3

Stepping (F10) performance has been improved on 59 already. “Opening the debugger” can be slow for various reasons, we are still working on improvements related to sourcemaps. If you have a link with steps to reproduce your issue, the best is to open a ticket on

Always worth trying Nightly, as we have been landing several performance improvements to the debugger in Firefox 59.

| I have another issue where a bunch of breakpoints keep coming back
| no matter how many times I remove them.

I thought most “breakpoints” issues were fixed before 57 :confused: . As a workaround you should be able to go to about:config and look for “devtools.debugger.pending-breakpoints”. Could you copy the value and report it here? After that you can right click and “Reset” the value. Hopefully it should remove those persistent breakpoints.

Hi, I’ll check out the Nightly .
I’d rather not have the contents of “devtools.debugger.pending-breakpoints” on a website. Can I send it to you privately?

I still have the issue with Firefox 59. A single tab opened with the debugger take 5Go of RAM.

I just created this bug report :


Jason Laster did a huge job to improve perf and it’s a lot better with Firefox 60a1, the current nightly build.

You can visit this repo if you want more about the debugger.


I’m able to reproduce, and working on a project that is open source, called GPU.js. I’d like to help solve this as it is the one hangup in firefox that has me switching back (temporarily) to other browsers.

To reproduce:

I can reproduce this 10 of 10 times on an i7 MacBook Pro. This is likely over-exasperating Firefox, because this function is somewhat highly recursive, but it will quickly exemplify the problem in a compounded way. Chrome, 10 of 10 times, has no issue whatsoever loading the debugger.

I hope this can help!

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Nearly two years later, and I’m still seeing this issue.