User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20180103231032 Steps to reproduce: This happens on every launch regardless of settings. I took the following steps to try and prevent it from happening: - disabling all addons - disabling automatic updates - disabling link prefetching and speculative pre-connections (both were disabled already) - disabling safebrowsing - disabling blocklist updating - disabling tracking protection - disabling sending telemetry, diagnostics, and crash reports - Firefox sync was not enabled to begin with Actual results: Every time I launch Firefox, it downloads about 80MB or so of data on my very limited 100 KB/s connection. This takes about 90 seconds each launch during which time anything else requiring network services is basically non-functional due to saturation. My first thought was that uBlock was downloading a bunch of filters, so I tried disabling all my addons but that didn't fix it. Process Monitor was showing a lot of things being written to the safebrowsing folder, so I tried disabling safebrowsing, but that didn't fix it. TCPView was showing most of the data was going to Cloudflare and about:networking was showing active connections to an image-heavy website I frequent that I think might use Cloudflare, which made me suspect prefetching, but that was already disabled by default. So I'm not really sure what's going on; I disabled about everything else I could think of and nothing seems to have fixed it. Expected results: I expect a certain amount of background data usage in most browsers for things like updates, telemetry, etc. But 80MB on each launch (and I suspect it might happen periodically after launch) seems excessive. Ideally, either this wouldn't happen, I would be able to control whether it happens, I'd be able to pinpoint *why* it's happening, and I'd be able to set bandwidth limits or prioritization on it so that it doesn't affect bandwidth usage of other applications and of normal website browsing within Firefox.
Can you please troubleshoot with Safe Mode: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode or even better, with a fresh profile: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
Component: Untriaged → Startup and Profile System
Product: Firefox → Toolkit
The issue persists using safe mode. The issue does *not* persist in a new profile. There's only a brief 1-2 second spike in traffic when starting Firefox using the new profile as opposed to a solid 90 seconds of saturation.
Interesting... so the website I mentioned that I saw connections to in about:networking that has a lot of images... I went to the website, blocked first party images using uMatrix, exited Firefox, and then restarted it. It seems to have fixed the issue. Tested it twice in case it was a fluke, but it's consistent. So it looks like it's prefetching data from the site in spite of the settings set up in about:config.
Created attachment 8941875 [details] Prefetch settings about:config prefetch and speculative pre-connect settings. Note that they're all disabled (I didn't do this, it was in this state when I checked). It seems these settings are being ignored and Firefox is prefetching data on launch anyway.
Is this site (the one from which data are downloaded) your homepage? It could explain why the data are loaded despite the prefetch preferences
The homepage setting is the default setting of Firefox start page, but I have Firefox set to load to a blank page.
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