It's still there in the 51.0.a1 nightly build.
There's a conversation on reddit about it (started by me) and also a bug report:
https://www.reddit.com/r/
firefox/comments/4nfmvp/ff_47_ unbearable_slow_over_remote_ x11/ https://bugzilla.mozilla.org/
show_bug.cgi?id=1263222
Stefan Seidel
------------------------------
------- On Sunday 28 August 2016 23:25:50 Stefan Seidel wrote:
> The variables gone by 50.Â~
>
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> On Mon, 29 Aug 2016, Stefan Seidel wrote:
> > Date: Mon, 29 Aug 2016 08:11:53 +0200
> > From: Stefan Seidel <sseidel@vub.de>
> > To: Jordan Sokolic <jordan@twiggle.com>
> > Cc: x2go-user@lists.x2go.org
> > Subject: Re: [X2Go-User] Web browser performance over X2Go
> >
> > Hi Jordan,
> >
> > Firefox 47 disabled XRender support by default. Go to about:settings and
> > search for xrender, set the property to true. That made it a lot better
> > for
> > me.
> >
> > Stefan
> >
> >> Date: Sat, 27 Aug 2016 12:34:32 +0300
> >> From: Jordan Sokolic <jordan@twiggle.com>
> >> To: x2go-user@lists.x2go.org
> >> Subject: [X2Go-User] Web browser performance over X2Go
> >>
> >> Message-ID:
> >> <CAFbFDtijPrfER_DEu+
zM11N08FKb_Zn- WdUEkR99AQN1zZ9o=Q@mail.gmail. com >> >>
> >> Content-Type: text/plain; charset="utf-8"
> >>
> >> Hello,
> >> I use a web frontend for work that runs locally on my server over X2Go
> >> for
> >> about one year with success.
> >> Some months ago I noticed a significant decrease in performance when
> >> using
> >> Firefox. My server is running Ubuntu 16.04 with MATE desktop, Windows 10
> >> client. Up to Firefox version 39.0.3 performance of loading pages,
> >> scrolling, browsing etc. was nearly flawless over a 5Mbit WAN link (with
> >> the exception of embedded video elements). However it seems the release
> >> of
> >> Firefox version 40.0 introduced some changes in rendering (see
> >> https://www.mozilla.org/en-US/
firefox/40.0/releasenotes/ ) that do not> >> play
> >> nice with X2Go. Further reduction in performance was noticed in versions
> >> 46.0 and 48.0. The current release (48.0) even has trouble rendering a
> >> plain text document without lag and choppiness.
> >> I have tried toggling various flags relating to graphics, compositing,
> >> rendering, etc. to no avail.
> >> To work around this problem the only solution I have found is to forward
> >> ports over a separate SSH tunnel and browse locally, but this is
> >> cumbersome
> >> to set up every time I need to connect.
> >> I have preferred Firefox to Chrome over X2Go because the latter has
> >> always
> >> been unusably slow; now they are both similarly choppy and slow.
> >> Has anyone else noticed this?
> >> Any advice or help would be appreciated.
> >>
> >> Regards
> >>
> >> Jordan