On the verge of being "fired" from post-doc position, how to act towards supervisor?
There are two problems here. An academic dilemma and a legal issue. The latter is specific to country's immigration, workplace rules, your residency status, etc., therefore I will politely keep the focus to the first one. There are also good comments to your post, so keep them in mind as well. Needless to say, the legal problem could be more intimate to your situation but you will find actual support from a legal authority as you yourself have stated. Now, to the academic dilemma at hand: first of all, I suggest talking with your supervisor and make your position clear in an honest sober way. Your problem is a practical one and need not to be emotional. Here, I would suggest and stress on keeping the best of relations with your supervisor (even if you decide to leave or they decide to let you go), especially if this is your first ever post-doc position. This is important for two reasons: first, you want to leave on good terms even if it didn't work out for you and so you don't need to give an impression of personal grudge. Second, there might be opportunities in future where you will have to work/collaborate with the said supervisor or the university. You also don't want to ruin the possibility of good references for your own future work from this university - hope things will work out for you, and my two cents help. Good luck.