(Reuters) - Cybersecurity firm FireEye Inc FEYE.O reported its first drop in quarterly revenue and forecast current-quarter billings well below analysts' estimates, sending its shares down as much as 20 percent.

Attendees walk by the FireEye booth during the 2016 Black Hat cyber-security conference in Las Vegas, Nevada, U.S. August 3, 2016. REUTERS/David Becker

FireEye, which provides web, email and malware security to businesses and governments, also said both its chairman and chief financial officer would leave, a setback for the company as it shifts its business to a cloud-based model.

Chairman David DeWalt, who stepped down as chief executive last year, led the company as it handled a string of high-profile data breaches over the last few years.

Chief Financial Officer Mike Berry is leaving the company to take up a similar role at another company. Frank Verdecanna, FireEye’s chief accounting officer, will replace Berry.

The company forecast revenue of $160 million-$166 million for the current quarter, missing the average analyst estimate of $176.6 million, according to Thomson Reuters I/B/E/S.

FireEye said it expects first-quarter billings - a closely watched indicator of future business - in the range of $130 million to $150 million, well below the average estimate of $186.3 million, according to research firm FactSet StreetAccount.

Vacancies in the company’s sales teams affected revenue in the fourth quarter, Chief Executive Officer Kevin Mandia said on a conference call.

Key positions, including heads of the worldwide and Europe sales teams, were vacant during the second half of last year, Mandia said. The positions have since been filled.

Billings fell 14 percent in the fourth quarter to $221.8 million, hurt by lower-than-expected product sales.

Analysts on average had estimated billings of $240.3 million, according to FactSet.

The company’s revenue fell for the first time since its market debut in 2013 to $184.7 million, missing the average estimate of $191 million.

Excluding items, FireEye reported a loss of 3 cents per share, beating the average estimate of a loss of 16 cents, helped by lower expenses.

Net loss attributable to shareholders more than halved to $61.5 million, or 37 cents per share, from $136.1 million, or 87 cents per share.