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Traders work on the floor of the New York Stock Exchange.

Check out which companies are making headlines before the bell:

Canadian Pacific — The firm sent a revised takeover bid to Norfolk Southern , saying it is "financially more attractive and dramatically reduces the regulatory uncertainty for NS shareholders." Norfolk's board of directors unanimously voted against the initial offer.

Toll Brothers — The luxury homebuilder poster mixed quarterly results, with earnings per share coming in at 80 cents and revenue at $1.44 billion. Analysts polled by Reuters expected the company to report earnings per share of 83 cent on revenue of $1.43 billion.

AutoZone — The auto parts retailer reported better-than-expected earnings per share for its fiscal first quarter. The company also posted revenue that was in line with expectations.

H&R Block — The tax service firm posted a wider-than-expected quarterly loss on Monday. The company said its total revenue fell 4.6 percent amid foreign exchange headwinds.



Staples — BB&T downgraded shares of Staples to "hold" from "buy," citing concerns over the Federal Trade Commission's decision to challenge its merger with Office Depot .

Amazon — Pacific Crest initiated coverage of Amazon's stock with an "overweight" rating and an $800 price target. "We believe that Amazon's ecosystem aims (we think successfully) to be a part of the consumer's purchase process for most anything, making it one of the stickiest retailers."

FireEye — Citi upgraded FireEye's stock to "buy" from "neutral," citing its ability to grow quickly and "expand its product line into adjacent markets."

Comcast — Nomura Securities initiated coverage of the media giant with a "buy" rating and a price target of $72 a share, citing its content ownership, brand distribution infrastructure, "advanced X1 user interface technology," and low price-to-earnings multiple. (Disclosure: Comcast owns NBCUniversal, which is the parent company of CNBC.)

Home Depot — The retailer reaffirmed its 2015 forecast, saying it expects sales to increase about 5.7 percent for the year, with same-store sales up approximately 4.9 percent. The company also expects adjusted earnings per share to grow approximately 14 percent to $5.36.