Firefox displays a confirmation dialog box with a short delay when you install an extension preventing you from mistakenly confirm the install providing adequate time to review the install information, this is a very good security feature and prevents several cleverly crafted arbitrary code execution holes.
However if you are an advanced user and would like to disable or decrease the install dialog box delay, follow the steps below:

Type about:config in Firefox address bar and press Enter.
Type security.dialog_enable_delay into the filter box, and hit ENTER as shown below.

Double-click and change the …

By default Firefox’s in-line spell checker does not work in all text-boxes and forms and you need to right-click and select “Spell check this field” to manually start spell-check, however this behavior can be automatically activated by following the steps provided below.

Type about:config in your Firefox address bar and press Enter.
Type layout.spell in filter text box.
Change the value to 2 as shown below.

Firefox only (Win/Mac/Linux): Firefox extension Download Manager Tweak customizes your downloading options, from docking the downloads window in the sidebar to changing the appearance.
Once you’ve installed the extension, head over to the Add-ons panel and open the Download Manager Tweak Options pane. From there, you’ll be able to customize just about everything in the downloads window, from the buttons that show up on the toolbar to setting a delay before the download manager opens. If you like controlling every aspect of your browsing experience, this is an add-on that might …