Here are your choices; 1) Contact its support and ask if they will update 2) Find a replacement 3) Downgrade to a prior Firefox 4) Do without

Do you mean Firefox Quantum: Developer Edition? Can be both installations created?

First off, the Developer Edition was merged into the Beta channel months ago; not the same as when Developer Edition was on the Aurora channel. All of the old Legacy extensions are incompatible with both the Release channel and Beta / DE channel versions. Only the Nightly alpha version allows for Legacy extensions, but there's no guarantee that they will work if they are enabled. And that's not a "problem", that's just the way it is.

As far as extensions that web developers may need and may use, better to tell us specifically which extensions that you are concerned about rather than throwing them all in the same basket; replacement WebExtensions may be available or may be in the process of being created to replace Legacy extensions.