My Mac screen has suddenly become too large, including the toolbar on top. It's as if it's stretched. I cannot correct, and now my Firefox screens don't seem to be "anchored" Very difficult to navigate? Thanks.

My screen suddenly became too large (kittens playing on keyboard). My top toolbar is stretched so that I can no longer see the items on the right (time, sound, etc. unless I scroll over). The resolution appears stretched, even though I've chosen the highest resolution. When I open Firefox or another application, that screen jumps around...it not longer is "anchored" to my top toolbar if that makes sense. Does anyone have a solution?

Thanks!

Computer information below:

Model Name: iMac
Model Identifier: iMac11,1
Processor Name: Intel Core i5
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 4
L2 Cache (per core): 256 KB
L3 Cache: 8 MB
Memory: 4 GB
Processor Interconnect Speed: 4.8 GT/s
Boot ROM Version: IM111.0034.B02
SMC Version (system): 1.54f36