With the Link Widgets extension installed, until iceweasel unstable
versions (10.x), a middle-click on a Link Widgets button opened the
page in a new tab, as expected. With iceweasel 18.0.1-1 from
experimental, the middle-click no longer does anything.

Note: I downgraded to 10.0.12esr-1, saw that the middle-click was
working, then upgraded to 18.0.1-1 again, and again the middle-click
on Link Widgets buttons no longer works.

I can reproduce the problem on two of my machines.

Control: retitle -1 iceweasel breaks Link Widgets: links not opened
Control: tags -1 upstream
Control: found -1 22.0-1

Now left clicks are affected too. I've seen this problem with
Firefox Nightly as well.

The URL that appears in the tooltip when putting the mouse over
a Link Widgets button is correct. So, the problem seems to be
related to opening the URL.

src:iceweasel has been superseded by src:firefox-esr in version
45.0esr-1 in March 2016. Transitional packages to ease upgrades were
provided in the wheezy, jessie, stretch and buster releases. The
transitional packages have been removed finally before the bullseye
release in August 2021.
After regular security support for buster ended in August 2022 and LTS
support ended in June 2024, I'm closing the remaining bug reports now.

Andreas