If you are using HTTPS everywhere on your site and set requireSSL="true" on your system.web/authentication/forms element in web.config, you are instructing the browser to only pass that cookie back over an HTTPS connection. This will protect against the vast majority of traffic sniffing-based session hijacking attacks and you should definitely use it if your site is HTTPS only.
Forms Authentication is inherently stateless. The server is encrypting the following information and storing it client-side: CookiePath, Expiration, Expired, IsPersistent, IssueDate, Name, UserData, Version. Assuming your machineKey hasn't been compromised, the client will just see this as a blob of encrypted data. When it presents that blob to the server again, the server decrypts it and converts it back into a FormsAuthenticationTicket, validates the fields in the ticket against config, verifies that the ticket isn't expired, etc. and decides whether to treat the request as authenticated. It doesn't 'remember' anything about which tickets are outstanding. Also note that it doesn't include the IP address anywhere.
The only real attack vector I can think of if you are HTTPS-only, take care to protect your machineKey, and set the forms auth cookie to requireSSL would be for an attacker to target the client's browser and/or computer. Theoretically they could steal the cookie from memory or disk out of the browser's space. It might be possible for a virus/trojan to do this or even a malicious browser extension. In short, if a user could get their hands on a valid, non-expired Forms Auth cookie, they could present it from any machine they wanted to until it expired. You can reduce the risk here by not allowing persistent auth cookies and keeping your timeouts to a minimum.
If they had the machineKey, they could create FormsAuth cookies from scratch whenever they wanted to.
Oh.. Can't forget Heartbleed. If you had a load balancer or reverse proxy that was using an insecure version of OpenSSL, it's possible an attacker could compromise your private key and intercept traffic over HTTPS connections. ASP.NET doesn't use OpenSSL, so you're safe from this in a pure-MS stack. If you ever hear anything about a vulnerability in MS' SSL implementation, you'd want to patch it ASAP and get your passwords changed and certificates re-issued.
If you are concerned about the browser/machine based hijacking, you might want to take a look at a project I started [and abandoned] called Sholo.Web.Security (https://github.com/scottt732/SholoWebSecurity). It's goal was to strengthen Forms Authentication by maintaining state on the server at the expense of some overhead on each request. You get the ability to do things like revoke tickets server-side (kick/logout a user) and prevent users from moving tickets between IP addresses. It can get annoying in the traveling mobile user scenario that Wiktor describes (it's optional). Feel free to fork it or submit pull requests.
The Anti-CSRF features that 0leg refers to apply to the UI/form mechanism that initiates the login process, but to my knowledge there is nothing in the Forms Authentication process itself that relates to CSRF. That is, once the cookie is issued to the client, the only thing protecting it from being bounced between servers is the fact that cookies are restricted to the domains/subdomain they were issued for. Your stackoverflow.com cookies won't be presented to serverfault.com. The browser takes care of that stuff for you.
