As a general rule, statements of opinion are not actionable as defamation. It’s also true, however, that indirect implications from a statement can form the basis of a defamation claim. When a person prefaces a statement with “in my opinion” or “I firmly believe,” it has no effect on whether the statement carries defamatory implications or innuendo. If a speaker expresses an opinion under circumstances that would cause a reasonable listener to understand that the opinion is based on the speaker’s knowledge of undisclosed facts, that “opinion” can be treated as an implied assertion of fact. And if that factual assertion isn’t true and conveys a defamatory meaning about someone, defamation liability may arise.
This isn’t really an exception to the rule that you can’t sue someone for defamation based on an expression of opinion. Pure opinions remain protected by the First Amendment. The question is whether a reasonable listener or reader would infer from a particular statement (whether couched as an opinion or otherwise) that the speaker or writer knows certain facts, unknown to the audience, which support the opinion and are detrimental to the reputation of the person about whom the statement is made. This is going to depend heavily on context, the identity of the parties, and the specific words used.