I've been laid off twice in a 20+ year (so far) coding career. It sucks, but it's usually nothing personal. Some angel investor money didn't come through, some sales contract didn't get signed, and now they have to cut staff. It happens. I've never been out of work more than a month.
Look at it from the company's side. They probably paid you a lot of money to build software for them. They may not have given you permission to GPL your code, or more likely they didn't understand or didn't care about the legal aspects of open source development. It may be hard to distinguish between the code your wrote on your own and the code you wrote for them, especially if you were a contractor working at home.
Emotionally, you should separate the circumstances of your leaving the company from the behavior of the company with respect to your GPL code. Suppose you had gotten a better job and left voluntarily. Would you feel the same way about your former employer using your open source code?
Consider approaching the company and say that you'd like to continue development on your toolkit as an open source toolkit. They'll probably agree. I live in a 4M pop metro area, and I am always running into people I worked with before. If you get a reputation for burning employers, it will come back to you eventually.