1. After getting GC if employer fires you within one month
will it be a problem for citizenship?
No, maintain clear documentation just in case.
2. After getting Green card is it good to change the
address before getting plastic card? Will the
address change may have any problems after passport
stamping and before getting plastic card?
You could always do that, all you need to do during your I551 stamping is to give them the new address. Also you need to send BCIS AR11. But having waited so long for your approval, I wouldn't suggest you take that risk (BCIS will most prob. send the card to your old address) of chaning your address.
3. If after getting GC you are with the same company
with no intent to leave. But the company is paying
'considerably' less than it said in LC document
can you sue your employer or somehow do
something to get same salary as mentioned in LC?
Do you really want to go down that path. Instead talk to them, raise your concerns, it can always be sorted out.