To a large extent, SeaMonkey extensions are also compatible with Firefox. The reverse is not always true.
I have 27 extensions (not plugins) installed. Here are my most important. Note that three are merely to restore capabilities that were lost when Mozilla developers decided that users really do not know what they need.
* Adblock Plus -- I do not subscribe to any filters; instead, I depend entirely on my own, manually-entered filters.
* Expire history by days -- Some developer at Mozilla decided that the users are wrong, that browser history should be pruned only when the database gets full. This extension restores that prior capability for users to set a preferred life-span for history entries. This extension was Firefox-only, but a Web tool allowed me to convert it for SeaMonkey.
* Find Preferences -- I hate the proliferation of banners in the user interface, another case where developers at Mozilla think they know what users need more than what the users say they need. This extension restores the prior capability to use a popup dialogue to search within a Web page.
* Flashblock -- Yes, I could use the Addons Manager to enable and disable the Flash plugin, Via the PrefBar extension (see below), Flashblock allows me to have a checkbox on my tool bar to enable and disable the Flash plugin without having to open the Addons Manager. Flashblock also indicates where on a Web page Flash presentations are present, provides a simple click to show the presentation, and a context menu to completely delete the presentation.
* Live HTTP headers -- I used this to find that my credit union was setting cookies for Facebook.
* Old Default Image Style -- Again, Mozilla developers decided that the user-set background color was not what users really wanted when displaying only a selected image. Instead, they forced a black background, which conflicts with images that have black along their edges. This extension restored the use of user-set background colors (pale mint green in my case).
* Password Exporter -- I use this to move passwords from my PC to my wife's. This extension was Firefox-only, but a Web tool allowed me to convert it for SeaMonkey.
* Passwords Button -- Part of the Toolbar Buttons extension (see below), this gives me a tool bar button to open the edit window of Password Manager so that I can delete, change, or copy passwords. This extension was Firefox-only, but a Web tool allowed me to convert it for SeaMonkey.
* PrefBar -- I want this as an inherent capability in the vanilla browser. I cannot easily browse without it. I have 31 checkboxes, buttons, and menus setup in PrefBar. Some are from the basic extension, some are added from the PrefBar Web site, and some I created myself.
* Secret Agent -- Although not entirely effective, this confuses attempts by Web servers to track me.
* Show Password On Input -- This is for my master password; see Show my Password below.
* Show my Password -- This is for login passwords. This and the Show Password On Input extension make passwords visible upon my request. I am getting old, and my fingers do not always type what I think I am typing. These let me see if I have mistyped a password and take corrective action.
* Theme Font & Size Changer -- This controls the fonts and their sizes in the browser's user interface, not on rendered Web pages. As I get older, I increase the sizes.
* Toolbar Buttons -- This provides an enhanced set of buttons for customizing my browser's tool bar. Additional buttons beyond that enhanced set are available from the extension's Web site.
Where I indicate "This extension was Firefox-only, but a Web tool allowed me to convert it for SeaMonkey.", the tool is at http://addonconverter.fotokrai....