How can I prevent Firefox from updating to a newer version [duplicate]

Since when going after the road-map, firefox 57 will be released on 14.11.207.

Now my problem is that I use a whole lot of legacy add-ons I want to keep working with.

So how can I prevent firefox from updating via the repositories?

You can do that with apt-mark. Open a terminal and use:

sudo apt-mark hold firefox

to pin firefox to the current installed version. Be aware of that you wont receive any updates for that package as long it is on hold this includes as well security updates which you will not get.

If you ever decide to want updates again simply use:

sudo apt-mark unhold firefox

If you want to check if you have hold back packages you can use:

sudo apt-mark showhold

For further information see man apt-mark.8