I encountered the exact same problem after installing Firefox 63 on two Windows 10 machines residing on my home network, and my online research found the following:

Found the reason for this behavior. For some reason the new version puts the proxy settings so that the "Auto-detect proxy settings for this network" is enabled. This makes it so that every page load it tries to first find a proxy, which takes seconds, and after that loads the page. Switched the setting back to No Proxy and everything is nice and fast again.

This seemed to be confirmed in a pair of official "bugzilla" reports:

Version 63 very slow performance, proxy set to "use system default" - fixed if set to "no proxy"

Firefox checks to determine proxy settings on every request - RESOLVED FIXED in Firefox 64

As a result, their recommended fix action is as follows:

Within Firefox, navigate to the about:preferences page
In the default General section view, scroll to the bottom to find the Network Settings area
Click on the Settings... button
In the subsequent window, select the radio button to modify the settings under Configure Proxy Access to the Internet to choose No proxy

However, although this solution apparently worked for several people, in my personal experience the problem persisted even after I changed my settings to No proxy. One of the Mozilla contributors recommended installing the Firefox 64 "Nightly" version to see if that eliminated the issue and it did for some users, but I'm not really a big fan of Beta software on my primary computers.

As an alternate solution, I uninstalled Firefox 63 on one of my computers, installed Firefox 60.3.0 ESR, and that alleviated the problem. Web page navigation and downloads were as fast as they were in Firefox 62. Also, please remember that a transition like this one is relatively painless, because your Firefox personal data is preserved in another location.

I left Firefox 63.0.1 installed on my secondary computer for ongoing testing purposes. When Firefox 64 is released to the general public in December, I will double-check to ensure that the problem has been alleviated.

Update

Mozilla has apparently addressed this issue in their latest update that increments the mainstream iteration of Firefox to version 63.0.3:

Run5k

Posted 2018-11-07T13:56:46.353

Reputation: 13 092

I tried this solution (no proxy), and I have the impression, that it is now slightly faster. Thank you. (I won't single"accept" this answer yet, though i upvoted it, because I'm searching for as many solutions as there are to get firefox page load faster :))

I also tried setting network.tcp.tcp_fastopen_enable to true. I think this also increased page load performance on some sites (less tcp overhead). Can you confirm that as well? – SearchSpace – 2018-11-07T20:19:10.803

@SearchSpace the No proxy solution, or installing Firefox 60 ESR, instead? – Run5k – 2018-11-07T20:22:20.983

No, you don't get notified when someone edits a comment... only when they are posted. That being said, as I mentioned within my answer, the No proxy solution didn't actually work for me, and there is a known issued with that particular version of Firefox. That's why I ultimately decided to drop back to the ESR version of the browser, and my downloads started working normally again. Prior to that, every-other-download would fail. – Run5k – 2018-11-07T20:25:00.650

Ah ok, thanks. First comment was edited. Btw. as I understand it: There are less chances to make load faster with more connections, because due to http2 the idea seems to be, to just have one connection, which transfers all data. I couldn't find out yet, if there are options to instruct firefox to load all elements parallely in this one connection (or if this is by default). Btw. do you have an powerful processor, does firefox utilize all your cores? – SearchSpace – 2018-11-07T20:30:09.707

Actually, both my laptop and desktop have mediocre hardware specs: an Intel quad-core i7 with 8 GB of RAM. Nothing special. – Run5k – 2018-11-07T20:31:41.170

I have the same problem on ff65, changing proxy settings to setting to no proxy also fixes the problem here – NZD – 2019-02-02T00:51:38.923

It has been several days since we heard from you. Did you have any lingering questions? – Run5k – 2018-11-28T21:11:16.870

@Run5k I'm not quite sure, if "no proxy" solved the problem. I'm waiting for Firefox 64 to see, if this solves the problem. It is released on 11. december. – SearchSpace – 2018-11-30T13:11:53.323

@SearchSpace if you take a look at the update within my answer, the latest Firefox patch to version 63.0.3 should have eliminated the problem. It did on the laptop that I utilize for testing purposes. – Run5k – 2018-11-30T13:34:09.523

@Run5k Ah, I didn't see that. Nice.I think it's a bit better. But I now assume that my problem is network bound. When I use F12 and look at the network analysis, it seems that my Firefox updates the Website in time with the frequests. With http/2 it should be better and I didn't found a setting which could enhance this (e.g. like pipelining in the early days). Prefetching astonishingly didn't make better side loads either (or i missconfigured??). – SearchSpace – 2018-11-30T17:29:43.420

Firefox 63 slow page load?

Answers

Update