For Photoshop and other heavy duty applications then i7 would be better choice. But the i5 is no mug either. Just get loads of RAM. 8GB with i5 or 8GB with i7! Normally i7 laptops are expensive. If it fits your budget, then why not. But you would need a good GPU to go along with it!
Also dont even count FF with 15 tabs.... that does not even break a sweat for these modern day CPU's! I have 3 FF windows open with 10-15 tabs in each and i am only on E5200 with 4GB RAM in my office!