I've become resigned to Firefox slowly leaking memory

July 19, 2017

Over the years I've written a number of things here about how my Firefox setup seems to be fairly fragile as far as memory usage goes, in that any number of addons or other changes seem to cause it to leak memory, often rapidly. Sometimes there have been apparently innocuous changes in addons I use, like NoScript, that cause a new version of the addon to make my Firefox sessions explode.

(I've actually bisected some of those changes down to relatively innocent changes and found at least one pattern in addon and even core Firefox JavaScript that seems to cause memory leaks, but I'm not sure I believe my results.)

For a long time I held out hope that if I only found the right combination of addons and options and so on, I could get my Firefox to have stable memory usage over a period of a week or more (with a fixed set of long-term browser windows open, none of which run JavaScript). But by now I've slowly and reluctantly come around to the idea that that's not going to happen. Instead, even with my best efforts I can expect Firefox's Resident Set Size to slowly grow over a few days from a starting point of around 600 to 700 MBytes, eventually crossing over the 1 GB boundary, and then I'll wind up wanting to restart it once I notice.

The good news is that Firefox performance doesn't seem to degrade drastically at this sort of memory usage. I haven't kept close track of how it feels, but it's certainly not the glaringly obvious responsiveness issues that used to happen to me. Instead I wind up restarting Firefox because it's just using too much of my RAM and I want it to use less.

(It's possible that Firefox's performance would degrade noticeably if I let it keep growing its memory usage, which of course is one reason not to.)

Restarting Firefox is not too much of a pain (I built a tool to help a while back), but it makes me vaguely unhappy despite my resignation. Software should be better than this, but apparently it isn't and I just get to live with it. Restarting Firefox feels like giving in, but not restarting Firefox is clearly just tilting at windmills.

Sidebar: The JavaScript pattern that seemed to leak

The short version is 'calling console.log() with an exception object'. The general pattern seemed to be:

try {
  [... whatever ...]
} catch (e) {

My theory is that this causes the browser-wide Firefox developer console to capture the exception object, which in turn presumably captures a bunch of JavaScript state, variables, and code, and means that none of them can be garbage collected the way they normally would be. Trigger such exceptions very often and there you go.

Replacing the console.log(e) with 'console.log("some-message")' seemed to usually make the prominent leaks go away. The loss of information was irrelevant; it's not as if I'm going to debug addons (or core Firefox code written in JavaScript). I never even look at the browser console.

It's possible that opening the browser console every so often and explicitly clearing it would make my memory usage drop. I may try that the next time I have a bloated-up Firefox, just to see. It's also possible that there's a magic option that causes Firefox to just throw away everything sent to console.log(), which would be fine by me.

Written on 19 July 2017.
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Last modified: Wed Jul 19 00:20:23 2017
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