Mozilla joined other browser vendors in announcing that Firefox will start blocking invisible Flash content this August, and that starting with 2017, it will block all Flash content by default as well.

Last year, Adobe, the creator of Flash, started encouraging Flash developers to switch over to using HTML for their content. However, the transition is not going to be a fast one until browsers start outright rejecting Flash content. Apple has already tried for almost a decade to kill Flash, and despite the popularity of its iOS devices, there are still many websites, especially those serving video content, that continue to use Flash players.

Many advertisers are also still reliant on Flash, which makes it difficult for websites to switch entirely to HTML. Google, Amazon, and Facebook began to push advertisers to HTML last year, while Chrome (and soon Microsoft’s Edge as well) has already started to auto-pause Flash ads, which also began last year. Chrome and Safari will both start to block most Flash content by the end of this year.

Mozilla said that Firefox would start blocking invisible Flash content next month that is not essential to the user experience. According to the company, this should reduce crashes and hangs by about 10 percent on average for its users. To maximize website compatibility, Mozilla will only block a curated list of Flash content that it can replace with HTML.

Over time, the list of content that Firefox can block will increase to the point where the browser will block all Flash content by default in 2017. That’s when Firefox will require “click-to-activate” approval from users for all Flash-based websites that not have switched to HTML.

In March 2017, Firefox will also completely drop support for all the NPAPI plugins, with the exception of Flash. The enterprise version of Firefox (ESR), which Mozilla will release the same month, will continue to support the Flash and Java plugins until 2018. Users who may still want to have Flash enabled by default could switch to Firefox ESR to use Flash for a while longer.

Flash has been the source of many critical browser exploits, so web security should improve significantly once major browsers do not support it anymore and most websites have made the transition to 100 percent HTML content.