Under the hood: How Firefox suggests tab groups with local AI

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Under the hood: How Firefox suggests tab groups with local AI

November 5, 2025

Rolf Rando & Vasish Baungally

Background

Mozilla launched Tab Grouping in early 2025, allowing tabs to be arranged and grouped with persistent labels. It was the most requested feature in the history of Mozilla Connect. While tab grouping provides a great way to manage tabs and reduce tab overload, it can be a challenge to locate which tabs to group when you have many open.

We sought to improve the workflows by providing an AI tab grouping feature that enables two key capabilities:

Suggesting a title for a tab group when it is created by the user.

Suggesting tabs from the current window to be added to a tab group.

Of course, we wanted this to work without you needing to send any data of yours to Mozilla, so we used our local Firefox AI runtime and built an efficient model that delivers the features entirely on your own device. The feature is opt-in and downloads two small ML models when the user clicks to run it the first time.

Group title suggestion

Understanding the problem

Suggesting titles for grouped tabs is a challenge because it is hard to understand user intent when tabs are first grouped. Based on our interviews when we started the project, we found that while tab groups are sometimes generic terms like ‘Shopping’ or ‘Travel’, over half the time users’ tabs were specific terms such as name of a video game, friend or town. We also found tab names to be extremely short – 1 or 2 words.

Generating a digest of the group

To address these challenges, we adopt a hybrid methodology that combines a modified TF-IDF–based textual analysis with keyword extraction. We identify terms that are statistically distinctive to the titles of pages within a tab group compared to those outside it. The three most prominent keywords, along with the full titles of three randomly selected pages, are then combined to produce a concise digest representing the group, which is used as input for the subsequent stage of processing using a language model.

Generating the label

The digest string is used as an input to a generative model that returns the final label. We used a T5 based encoder-decoder model (flan-t5-base) that was fine tuned on over 10,000 example situations and labels.

One of the key challenges in developing the model was generating the training data samples to tune the model without any user data. To do this, we defined a set of user archetypes and used an LLM API (OpenAI GPT-4) to create sample pages for a user performing various tasks. This was augmented by real page titles from the publicly available common crawl dataset. We then used the LLM to suggest short titles for those use cases. The process was first done at a small scale of several hundred group names. These were manually corrected and curated, adjusting for brevity and consistency. As the process scaled up, the initial 300 group names were used as examples passed to the LLM so that the additional examples created would meet those standards.

Shrinking things down

We need to get the model small enough to run on most computers. Once the initial model was trained, it was sampled to a smaller model using a process known as knowledge distillation. For distillation, we tuned a t5-efficient-tiny model from the token probability outputs of our teacher flan-t5-base model. Midway through the distillation process we also removed two encoder transformer layers and two decoder layers to further reduce the number of parameters.

Finally, the model parameters were quantized from floating point (4 bytes per parameter) to integer 8 bit. In the end this entire reduction process reduced the model from 1GB to 57 MB, with only a modest reduction in accuracy.

Suggesting tabs

Understanding the problem

For tab suggestions, we identified a couple of approaches on how people prefer grouping their tabs. Some people prefer grouping by domain to easily access all documents for work for instance. Others might prefer grouping all their tabs together when they are planning a trip. Others still might prefer separating their “work” and “personal” tabs.

Our initial approach on suggesting tabs was based on semantic similarity. Tabs that are topically similar are suggested.

Identifying topically similar tabs

We first convert tab titles to a feature vector locally using a MiniLM embedding model. Embedding models are trained so that similar content produces vectors that are close together in embedding space. Using a similarity measure such as cosine similarity, we’re able to assign how closely similar a tab title or url is to another.

The similarity score between an anchor tab chosen by the user and another tab is a linear combination of the candidate tab with the group title (if present) of the...