I work for a Fortune 50 that sells a product many would describe as slow and bloated. The former is true because of the latter, and the latter is true because of demand. When the product was younger it was sleek and fast. As a product tacks on features it necessarily becomes slower to accommodate them. There are possibly exceptions to that rule, but not many. Browsers are not an exception.
Either the software initializes the subsystem to support a feature at startup, or (as they usually try to do) it's initialized in the background. Unfortunately someone will come up with a killer use for that feature (either internally or externally) that requires support during startup. Invariably that means startup is slower. Tack on intricate and complex dependencies, eventually everything gets initialized at startup. Not to mention that once you ship a product, those APIs are officially supported, no matter how well they were thought out or how early they were introduced.
The perceived result is bloatware, yet there's nothing in the product except features that were demanded by the users. Firefox certainly isn't perfect, and I'm sure the developers would love to rewrite portions of the product, but most of those rewrites will gain maintainability rather than performance. Performance is usually something you squeeze out of what's already there, and both Chrome and Mozilla engineers are very capable at spotting opportunities.