Sequential Tab Loader

Sequential Tab Loader attempts to solve a particular problem - when opening many new tabs quickly, Firefox handles that... very poorly. It attempts to load them all at once, slowing down the browser and extending the time until any tabs load at all. Especially bad on a slow internet connection.

This addon does the following:

• Makes new tabs load in unloaded (discarded) state first, to minimise resource usage
• Adds the newly opened unloaded tabs to a queue
• Loads tabs from the queue one by one by default, or in batches if you specify that in Options
• Allows you to set a max limit on the loading queue - when new tabs are opened exceeding said limit, they will remain in a discarded state until you click them.
• Allows you to (re)load existing selected tabs sequentially using the same queue system

Usage instructions:

• the addon is active by default and can be used as is, with its default settings.
• It is highly recommended to pin the addon icon to the toolbar, this way you will be able to see how many tabs are in the queue waiting to be loaded.
• clicking the addon icon pauses/unpauses addon operation. Pausing it will not clear the queue, after unpausing it will continue loading from where it left off. However, new items aren't added to the queue when the addon is paused.
• If you want to load / reload already opened tabs - select them, right click on any of the selected tabs -> "Reload Tabs (Queued)". This command will add the selected tabs to the addon's queue to be loaded sequentially, as opposed to using Firefox's regular "Reload Tabs" option, which will reload everything at once.

Options menu

• Maximum concurrent tabs to load - how many tabs are being loaded at once. One by default. Increase if you want to load them in batches instead.
• Queue limit - max number of tabs that can be added to the loading queue. When this limit is exceeded, further opened new tabs will just remain unloaded after being opened. If you need to open many links, but not necessarily need all of them to be loaded, limiting the queue to a smaller number can achieve that.
• Tab loading behaviour - "Load tabs" is the default mode of loading tabs from a queue. If you choose "Don't load tabs" - it will disable the loading queue, and all newly opened tabs will remain in an unloaded (discarded) state until clicked. Can be used if you want to open many links, but don't want them to be loaded automatically.
• Tab discarding delay - delay after the tab is created and before its unloaded. Normally not needed.
• Tab loading delay - delay after the previous tab finishes loading and before the next one starts loading. Setting this delay will disable "Maximum concurrent tabs to load" option and tabs will be loading one by one. Some sites can stop pages from loading until you complete a captcha if you attempt to open too many links in a short time. If you encounter that, increasing this setting to like 500ms or more can prevent that from happening.
• Alt+click links behaviour: none - nothing happens, the addon doesn't change anything.
• Alt+click links behaviour: Open tabs as discarded (sequential tab loading) - rather than trying to intercept regular tabs and discard them, creates tabs in a discarded state right away, which is faster and the tabs are guaranteed to get their URL that way. Can also work on some links where Ctrl+click fails. Disadvantage - in some rare cases might not work on some links that Ctrl+click works on.
• Alt+click links behaviour: Add links to tab creation queue (sequential tab creation) - rather than creating all tabs immediately and then loading them via a queue, the addon only creates the next tab after the previous one finishes loading. This is handled by a separate tab creation queue, indicated by orange background on the addon's toolbar icon when its active.
• Drop duplicate URLs from tab creation queue - if the URL being added is already in the queue, it won't be added again, to prevent opening duplicate tabs.
• Enable tab ignore signal listener - if you have a bookmarklet that opens tabs, those likely will end up blank if added to the loading queue. With this option enabled, it can send a signal to ignore these tabs, so that they could load correctly. To send a signal, use the following code to ignore 1 tab: "document.dispatchEvent(new CustomEvent('STL_IgnoreNextTab'));" or this to ignore a specified number of tabs, 3 in this case: "document.dispatchEvent(new CustomEvent('STL_IgnoreNextTab',{detail:{count:3}}));"

• "Resume loading" - resumes tab loading / enables the queue. Can also be used in case the queue got stuck and stopped loading tabs.
• "Load next tab" - this option will skip the current tab and start loading from the next one
• "Empty the queue" - clears the loading queue entirely, tabs will remain unloaded until clicked. Tab creation queue will also be cleared, so any unopened URLs there will be lost.
• "Keep tabs discarded (disable queue)" - disables the queue to just open tabs in a discarded state without them being loaded. To resume the loading / enable queue, select "Resume loading".
• "List pending creation queue URLs" - when there are still unopened URLs in the tab creation queue, they will be listed in a new tab.

• Make bookmarks open as background tabs, so that they could also be loaded via a queue. Go to "about:config" page, set "browser.tabs.loadBookmarksInBackground" to "true"
• Go to Firefox Settings -> Tabs section -> disable "When you open a link, image or media in a new tab, switch to it immediately" - you don't want focus constantly switching to the new tabs, as this will trigger their immediate loading, invalidating the loading queue.

• In some rare cases the queue can get stuck and stop loading tabs - in that case a safeguard timer should attempt to resume loading from the next tab/URL. If that didn't work, try the addon icon context menu options - "Resume loading" or "Load next tab".
• When you have the following about:config property: "browser.tabs.loadDivertedInBackground" set to "true", and you left click a link that opens a new tab, that tab opens blank. Opening the same link from its context menu, or by Ctrl+click, or with middle mouse button click works just fine. Why? ¯\_(ツ)_/¯ FF is weird. So far I haven't found how to deal with such links without affecting all the other ones. If you encounter this - set the tab discarding delay. Try the 50ms first, and if you still see blank tabs, increase the delay until tabs start getting their URLs. Alternatively, you can open links with "Alt+click links to open in discarded tabs" feature - this avoids the described problem entirely, since the addon itself will handle creating new tabs.

• Access your data for all websites (<all_urls>) - needed for discarding (browser.tabs.discard) and reloading (browser.tabs.reload) tabs to work on all URLs. Without said permission the addon would only be able to work on pre-defined domains.
• Access browser tabs - needed to interact with tabs, listen to tab events (creation, updates, removal) and get tab properties. Without it the addon would be non-functional.

If you found any bugs/issues, or have ideas about adding some new functionality (as long as its relevant), you can open a new issue following the Support Site link.