OP: you have a great opportunity to negotiate. Keep in mind that nobody is indispensable at any company. Money is not all, but how it helps!
The whole package is the key: ask for more benefits, a better severance package, more vacation days, more bonuses, etc...You must show why you deserve more, with hard facts, numbers, not only time in the company.
The salary of new hires is always based on the budget available to cover that position, and how difficult is to find someone with the particular skills, knowledge and experience to do the job.
Market value of someone like you is always more than what you rcurrenlty receive.
For companies to find someone always cost more than what they currenlty pay to similar employees, this is based on the fact that if it is for a specialized position they will need to hire from a competitor, and for the candidate to make the move there should be an incentive. Or hire someone with potential that requires training (money or other type of investment = time).
However with the current economy and the massive amount of people unemployed many companies take advantage and find people that may not be a perfect fit for what is needed, and they prefer to pay less and use the current experienced employees to coach and train them.
Even you have more time in the company, that doesn't mean that you deserve more money than a new hire, sometimes new hires are more talented or have specific knowledge, skills or experience than people that have decades working there.
Changing roles or positions within the same company or working for another one is the easiest way to get more money, and many times less responsabilities.