It seems there is a chance this could be coming… Rep Jackie Speier has said there is a rumor that he might try to do it over Christmas 
Watch: Intel Dem warms of Trump plot to fire Mueller on Christmas weekend
Here is WTF you can do…
If this does happen either soon or anytime in the future - there is already a rapid response protest structure in place. You can find a local protest at MoveOn here: Nobody Is Above The Law
Rallies will begin hours after news breaks of a Mueller firing (all times are LOCAL time):
- If Mueller is fired BEFORE 2 PM: events will begin @ 5 PM
- If Mueller is fired AFTER 2 PM: events will begin at noon the following day
This is the general plan—confirm details on the specific event page, as individual hosts may tailor their events to their local plan.
This could happen at any time, take advice from Walter Shaub (former Director, Office of Govt Ethics) and be prepared! 
From MoveOn.org:
Donald Trump is publicly considering firing special counsel Robert Mueller, the person leading the Department of Justice investigation of possible illegal actions by Donald Trump and members of his presidential campaign, and the efforts to conceal those activities.
This would be a constitutional crisis for our country. It would demand an immediate and unequivocal response to show that we will not tolerate abuse of power from Donald Trump.
Our response in the minutes and hours following a power grab will dictate what happens next, and whether Congress—the only body with the constitutional power and obligation to rein Trump in from his rampage—will do anything to stand up to him.